Bach - but not as he wrote it
'Reverse engineering' music the admired was a key feature of the Lennon-McCartney songwriting methodology. Please Please Me, for example, was initially Lennon's attempt at producing a typical Roy Orbison ballad. It does not sound like the Big O on the final recorded version but that is where it started.
Similarly, though the Bach connection may not be immediately apparent in Blackbird, it is there in the musical scaffolding
Part of its structure is a particular harmonic thing between the melody 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... 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.
Barry Miles, Paul McCartney: Many Years From Now (page 485)