Black magic is similar to selling one's soul, a magical practice colorfully passed down through the centuries in the Faust legends and literature.
Christopher Marlowe (1564-1593), Johann Goethe (1749-1832), and Thomas Mann (1875-1955) all told the Faust story. The stories, though distinct and expressing the writers' different philosophies, revolve around the same theme: A man willing to give his soul in exchange for earthly gain. In Marlowe, Faust makes a pact with the devil (Mephistopheles) in order fulfill his immoderate ambitions–to conquer Africa and Spain, to be the great emperor of the world, to make a bridge over the ocean. When the devil comes to claim his soul, Faust cannot bring himself to repent of his error because repentance would mean renouncing his power. The devil drags him off shrieking and physically tears him apart. In Goethe, Faust's desire to understand the secrets of the universe (a desire which had, significantly, already moved him to experiment with the occult) motivates him to make a pact with Mephistopheles. Goethe's Faust is saved at the end because he learns to love. In Mann, Faust (named Adrian) makes a pact with the devil in which he gives up joy and love in return for success as a composer and communion with the deeper powers. At the end he dies a madman ravaged by syphilis.
Soul-selling stories like the Faust legends are fascinating because the protagonist is so twisted that he is willing to barter that which is of infinite value (his soul) in exchange for finite goods (earthly success). It's very similar to the black magician. His desire for earthly goods is so warped that he is willing to comport with the devil, evil and ugliness; it's a price totally disproportionate to the potential gains.