"Eugene's Trick Bag", the updated classical piece at the film's climax, is largely based on Niccolò Paganini's "Caprice #5". According to myth, Paganini sold his soul to the devil for his musical skills. Steve Vai replicates Paganini's legendary rolling eyes, long unkempt hair and gaunt look.
The script was an original by John Fusco, who had long been interested in blues music. He worked as a blues singer and musician, but had been warned to rest his vocals by a doctor. In 1981, his girlfriend, who was working at a rest home, told him that an old black man with a harmonica had been admitted. Fusco went to visit him, and on the way, dreamt up a story about what would happen if the player was a legendary blues player. This gave him the idea for the story.
As the film opens Robert Johnson is seen playing and singing, with a high-pitched soprano voice. Allegedly, he actually had a deeper voice--when his recordings were made the speed of the master was slowed down because Johnson's songs were so long they would not "fit" on the recorder, so slowing the device would collect more but raise the pitch when played back. That's why all of his original 78-rpm records play back at a higher pitch than what he actually sang. Modern digital technique allows these recordings to be played back at the true and correct pitch with the speed slowed down, which drops his vocal range back to his real one and the authentic speed value.
Though the blues guitar sounds aren't truly coming from Ralph Macchio's fingers, he plays the music of Steve Vai and Ry Cooder note for note. His fingering, slides and bends are precise until the "main" solo, which incorporates Niccolò Paganini's "5th Caprice", where he does not finger the correct locations on the guitar. He mostly uses the same pattern (scale on the top frets, then another one in the bottom frets). The scales shift and change sound, but his patterns remain the same.