The Magical Negro is typically but not always "
in some way outwardly or inwardly disabled, either by discrimination, disability or social constraint," often a janitor or prisoner.[6] He has no past; he simply appears one day to help the white protagonist.[7][8] He usually has some sort of magical power, "rather vaguely defined but not the sort of thing one typically encounters."[7] He is patient and wise, often dispensing various words of wisdom, and is "closer to the earth."
http://en.wikipedia.org/wiki/Magical_Negro