Paul McCartney explained on Chaos and Creation at Abbey Road, aired in 2005, that the guitar accompaniment for "Blackbird" was inspired by Johann Sebastian Bach's Bourrée in E minor, often played on the classical guitar. As teenagers, he and George Harrison tried to learn Bourrée as a "show off" piece. The Bourrée is distinguished by melody and bass notes played simultaneously on the upper and lower strings.
McCartney adapted a segment of the Bourrée (reharmonised into the original's relative major key of G) as the opening of "Blackbird", and carried the musical idea throughout the song.
en.wikipedia.org/wiki/Blackbird_(Beatles_song)#Origins