"Baby in Black" performs at a 6/8 time signature with a moderate tempo that makes it sound like 3/4 (waltz-time). AMG described the song as "a love lament for a grieving girl that was perhaps more morose than any previous Beatles' song." Musicologist Alan W. Pollack notes that the song is relatively complex in format, with a refrain, bridge, and a guitar solo. He describes the song as having "mishmash" of sylistic elements—among them, "bluesy" chords and country music-inspired vocals.
"Baby's in Black" was recorded by the Beatles on 11 August 1964, and was the first song recorded for Beatles for Sale. Lennon and McCartney sang their vocal parts simultaneously through the same microphone. This was done at their own insistence in order to achieve a closer feel to the performance. McCartney was subsequently contacted by their music publisher in 1964 inquiring as to which melody line was the main tune (i.e. Paul's higher or John's lower melody). McCartney later said that he told the publisher they were both the main melody.
They performed "Baby's in Black" live during their appearances from late 1964 until their last tour in 1966. McCartney said they introduced the song by saying, "'And now for something different.' ... We used to put that in there, and think, 'Well, they won't know quite what to make of this, but it's cool.'". In 1996, a live version of "Baby's in Black" was released as a B-side to "Real Love", the second single from their Anthology project.