On June 11 1968, Paul McCartney sat at EMI Studio 2 and was filmed as well as recorded performing his new song "Blackbird" featuring himself and an acoustic guitar. John and Yoko as well as Paul's current girlfriend Francie Schwartz were all present at the studio as well.
After performing a run-through of the song for producer George Martin (who was also present), Paul started recording once Mr. Martin had gone up the stairs of the studio to the control room. It was suggested that Paul rehearse the song and record it as a "demo" in order to tighten up the arrangement. In the end, a total of 32 "takes" were recorded on tape in the studio. At one point, the idea of a string quartet in the second verse is discussed as well as a suggestion from John Lennon suggesting a brass band (which would be used eventually on another McCartney composition: "Mother Nature's Son").
Finally, the official takes begun and take 4 was used on the Apple 3-LP/2-CD "Anthology 3" project whose main difference from the commercial release is that the first verse is repeated. In the end, take 32 was chosen as the best take and McCartney double tracked his voice in some places. Six mono mixes were made and a temporary mix (the last one) was kept for almost four months until the official mono and stereo mixes were completed on October 13 1968. The sound effects were added during the October mixing sessions and the difference between the official mono and stereo mixes are the timings for the bird effects in the song (sooner in mono).
The take 4 version of the song can be heard on such CD bootlegs as "Unsurpassed Masters Vol. 4" and "The Peter Sellers Tape". Most of the rehearsal takes can be heard on the bootleg CD "Gone Tomorrow, Here Today" which is a Paul improvisation sung during the rehearsal sessions.