 Once "In Rock" had been completed, EMI asked for a suitable single to be recorded to help promote the album.Though Roger Glover states that Ricky Nelson's 1962 hard rocking arrangement of the George Gershwin song "Summertime" was the basis for the Mk II Deep Purple single "Black Night", it is also similar to Blues Magoos's 1966 psychedelic hit song "(We Ain't Got) Nothin' Yet".In the BBC documentary "Heavy Metal Britannia", keyboardist Jon Lord supports Glover's statement about the song's origin, stating "Black Night was nicked from the bass line in Ricky Nelson's Summertime" and then proceeds to play the bassline riff on his grand piano.