Paul McCartney has cited Johann Sebastian Bach as an influence on "Blackbird". He admitted in 1997: "The original inspiration was from a well-known piece by Bach, which I never know the title of, which George and I had learned to play at early age; he better than me actually. Part of its structure is a particular harmonic thing between the meldoy and the bass line which intrigued me. Bach was always one of our favourite composers; we felt we had a lot in common with him. For some reason we thought this music was very similar to ours and we latched on to him amazingly quickly. We also liked the stories of him being the church organist and wopping this stuff out weekly, which was rather similar to what we were doing. We were very pleased to hear that. I developed the melody on guitar based on the Bach piece and took it somewhere else, took it to another level, then I just fitted the words to it." (Miles page 485)

Paul has forgotten the Bach piece's title, but many fans and scholars have assumed it's the Bourrée from his E Minor Lute Suite, BWV 996.

The only similarity I hear by listening is that they're both played on acoustic guitars. (But even then, the guitar Paul played on "Blackbird" had metal strings, whereas classical guitars typically use nylon strings to give a warmer sound - and the above recording is no exception).

Perhaps score study will yield results.

The Bourrée is in E minor while "Blackbird" is in G major - so they share the same key signature of one sharp - and both use highly chromatic harmony, but there is no clear relationship. This makes me wonder why so many have assumed the Bourrée to be the correct Bach piece. Might it be some other?

As it turns out, no, it is the Bourrée. Paul might not know the name of it, but he can still play it, and on 28 July 2005, Paul performed a live solo concert at Abbey Road Studios, which was recorded and released as Chaos and Creation at Abbey Road. During the performance, he again cited Bach as an influence on "Blackbird" and even played a few measures of the Bach original to illustrate the similarities (which can be heard from about 37:39-38:51 in the video below).

This proves that Paul was indeed thinking of the Bourrée in E minor when composing "Blackbird". However, Paul actually plays it wrong when demonstrating. He even acknowledges that the way he plays it, saying, "it really should go [sings it correctly], but we didn't know that bit." The version Paul played, then, is transcribed below (and transposed from D minor up to E minor for the sake of comparison).

Paul further admitted that "the bit where we went wrong, I later took that, the idea that you get the bass line and the top line (the melody) and turned it into 'Blackbird'." And now it makes sense! Comparing the Bourrée (as Paul played it) and "Blackbird" side by side, we can easily see the similarity. The example below shows in blue notes that are identical between the two, while red notes indicate comparable (if not identical) similarities. Further notice that Paul adds several notes (the black ones). 

And the rest of "Blackbird" uses similar harmonies - parallel tenths with a pedal G on the off beats. Those parallel tenths are only found in Paul's version of the Bourrée. Thus, Paul definitely was influenced by Bach's Bourrée in E minor  when writing "Blackbird", but by his own version of the Bourrée rather than the Bach original.


CITATIONS
Miles, Barry. Paul McCartney: Many Years From Now. Henry Holt and Company Inc., New York, NY 1997.

Given the novel structure of [127] "Revolution 9", I am temporarily skipping it and jumping ahead to [128] "Blackbird":

Formal structure of "Blackbird":
          Intro (v)    0:00-0:04
          Verse 1      0:04-0:28
                    Ext         0:23-0:28
          Verse 2      0:28-0:47
          Middle 8    0:47-0:58
          Break (v)   0:58-1:16
          Middle 8    1:16-1:28
          Break (v)   1:28-1:47
          Verse 3      1:47-2:05
          Coda (ext of verse)    2:05-2:18

Comments: "Blackbird" is yet another Beatles song to use contiguous verses (following [1] "Love Me Do", [7] "Do You Want to Know a Secret", [8] "Misery", [9b] "Anna (Go To Him)", [9c] "Boys", [9d] "Chains", [9f] Twist and Shout, [10] "From Me To You", [13e] "Till There Was You", [17] "Little Child", [19] "Not a Second Time", [23] "Can't Buy Me Love", [25] "And I Love Her", [26] "I Should Have Known Better", [28] "If I Fell'', [29] "I'm Happy Just to Dance With You", [31] "A Hard Day's Night", [31b] "Matchbox", [32] "I'll Cry Instead", [35] "Things We Said Today", [40] "I Don't Want To Spoil the Party", [41] "What You're Doing", [42] "No Reply", [43] "Eight Days a Week", [44] "She's a Woman", [44b] "Kansas City/Hey Hey Hey Hey", [46d] "Words of Love", [47] "Ticket to Ride", [49] "I Need You", [50] "Yes It Is", [51] "The Night Before", [52] "You Like Me Too Much", [54] "Tell Me What You See", 56b] "Dizzy Miss Lizzy", [56c] "Bad Boy", [57] "I've Just Seen a Face", [59] "Yesterday", [66] "If I Needed Someone", [68] "We Can Work it Out", [71] "Michelle", [77] "Tomorrow Never Knows", [80] "Paperback Writer", [82] "Doctor Robert", [84] "Taxman", [88] "Yellow Submarine", [89] "I Want To Tell You", [92] "She Said She Said", [95] "Penny Lane", [96] "A Day in the Life", [99] "Fixing a Hole", [100] "Only a Northern Song", [101] "Being for the Benefit of Mr. Kite", [105] "Within You Without You", [111] "All Together Now", [114] "All You Need Is Love", [116] "I Am the Walrus", [122] "Lady Madonna", and [126] "Don't Pass Me By"), however in this case there is an extension separating the verses - something not found in any other song to date. If you count the breaks as verses (they use the same music, just without vocals), then the last break and last verse are also contiguous verses.

[121] "The Inner Light"
The middle 8 is used twice, lasting 8 or 9 measures and 23 or 18 seconds each time (the second iteration transitions to the coda, accounting for the slightly shortened duration), totaling 26.3% (41/156) of the song's duration.

[122] "Lady Madonna"
The middle 8 is used thrice (with the middle of those three replacing vocals with a saxophone solo), lasting 8 measures and 18 or 17 seconds each time, totaling 38.7% (53/137) of the song's duration. The middle 8s also modulate from A Major to C Major.

[123] "Across the Universe"
"Across the Universe" is the 46th Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[124] "Hey Bulldog"
"Hey Bulldog" is the 47th Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[125] "Revolution 1"
"Revolution 1" is the 48th Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[126] "Don't Pass Me By"
"Don't Pass Me By" is the 49th Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[127] "Revolution 9"
"Revolution 9" is the 50th Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[128] "Blackbird"
The middle 8 is used twice, lasting 4.5 measures and 11.5 seconds each time, totaling 16.7% of the song's duration.

[129] "Everybody's Got Something to Hide Except for Me and My Monkey"
"Everybody's Got Something to Hide Except for Me and My Monkey" is the 51st Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[130] "Good Night"
The middle 8 is used thrice, lasting 4 measures and 14 or 15 seconds each time, totaling 22.3% of the song's duration.

[131] "Ob-La-Di Ob-La-Da"
The middle 8 is used twice, lasting 8 measures and 15 seconds both times, totaling 16.0% of the song's duration. The middle 8 tonicizes IV.

[132] "Revolution"
"Revolution" is the 52nd Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[133] "Cry Baby Cry"
"Cry Baby Cry" is the 53rd Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[134] "Helter Skelter"
"Helter Skelter" is the 54th Beatles original not to use a middle 8. Instead, the chorus serves as contrast to the verses.

[135] "Sexy Sadie"
The middle 8 is used twice, lasting 5.5 measures and averaging 11.5 seconds each time, totaling 11.9% of the song's duration.

[136] "While My Guitar Gently Weeps"
The middle 8 is used twice, lasting 16 measures and 34 seconds each time, totaling 23.69% of the song's duration. The middle 8 modulates from A minor to A major.

[137] "Hey Jude"
The middle 8 is used twice, lasting 11.5 measures and 37.5 seconds each time, totaling 17.5% of the song's duration.

[138] "Mother Nature's Son"
The middle 8 is used twice, lasting 7.5 measures and 21 seconds each time, totaling 25.1% of the song's duration.

[139] "Yer Blues"
The middle 8 is used thrice, lasting 4 measures and averaging about 10 seconds each, totaling 12.1% (29/239) of the song's duration.

[140] "Rocky Raccoon"
The middle 8 is used twice, lasting 8 measures the first time and 9 measures the second (because the second middle 8 concludes the song), averaging 24 seconds each, which totals 22.6% (48/212) of the song's duration.

[141] "Wild Honey Pie"
"Wild Honey Pie" is the 55th Beatles song not to use a middle 8. The novel structure of the song has no need for a middle 8.

[142] "Back in the USSR"
The middle 8 is used twice, lasting 10 measures and 17 seconds each time, totaling 14.7% (24/163) of the song's duration.

[143] "Dear Prudence"
The middle 8 is used once, lasting 5.5 measures and 17 seconds, totaling 7.2% (17/235) of the song's duration.

[144] "Glass Onion"
The middle 8 is used once, lasting 10 measures and 19 seconds, totaling 13.9% (19/137) of the song's duration. This middle 8 concludes with two bars borrowed from the end of verse B.

[145] "I Will"
The middle 8 is used once, lasting 8 measures and 19 seconds, totaling 18.1% (19/105) of the song's duration.

[146] "Birthday"
The middle 8 is used twice, lasting 10 measures and 14 seconds each time, totaling 17.3% (28/162) of the song's duration. The middle 8 modulates form A major to C major.

[147] "Piggies"
The middle 8 is used once, lasting 7 measures and 18 seconds, totaling 14.6% (18/123) of the song's duration.

[148] "Happiness is a Warm Gun"
"Happiness is a Warm Gun" is the 56th Beatles song not to use a middle 8. The novel structure of the song has no need for a middle 8.

[149] "Honey Pie"
The middle 8 is used twice, lasting 8 measures and 13 seconds each time, totaling 16.3% (26/160) of the song's duration.

[150] "Savoy Truffle"
The middle 8 is used twice, lasting 8 measures and averaging 15.5 seconds, totaling 21.1% (31/174) of the song's duration.

[151] "Martha My Dear"
The middle 8 is used twice, with the first iteration incorporating a substantial extension. The first middle 8 (extension included) is 15.5 measures and 42 seconds long; the second is 8.5 measures and 23 seconds long. Combine these two, and the middle 8 totals 43.9% (65/148) of the song's duration.

[152] "Long Long Long"
The middle 8 is used once, lasting 7.5 measures and 26 seconds, totaling 14.1% (26/185) of the song's duration.

[153] "I'm So Tired"
"I'm So Tired" is the 57th Beatles song not to use a middle 8. Instead, the chorus serves as a contrast to the verses.

[154] "The Continuing Story of Bungalow Bill"
"The Continuing Story of Bungalow Bill" is the 58th Beatles song not to use a middle 8. Instead, the chorus serves as a contrast to the verses.

[155] "Why Don't We Do It In The Road"
"Why Don't We Do It In The Road" is the 59th Beatles song not to use a middle 8. (There is no contrast to the verses, with the possible exception of the percussion introduction).

[156] "Julia"
The middle 8 is used once, lasting 5 measures and 18 seconds, totaling 10.2% (18/176) of the song's duration.


Updated cumulative chart: