Technically speaking, it should be “magical negro”, but saying “mystical negro” makes it a Fast Lane reference, and that trumps accuracy any day. In any case, the archetypal or stereotypical mystical negro is, according to WikiPedia, “someone uneducated and in a low station of life, such as a janitor or prisoner. The black character is depicted as wiser and spiritually deeper than the protagonist, and the purpose of the “Magical Negro” in the plot is often to help the protagonist get out of trouble, and to help the white character recognize his own faults and overcome them.”