There's an interesting list over at The Onion AV Club of "13 Movies featuring magical black men". You know the type: benevolent Christ-figures who give their all to an undeserving white person, teaching them how to dance and do kungfu (think Morpheus), never asking anything in return.
When I went to Rocky Point High School in New York, I remember reading a preponderance of books that were basically high-minded variations on this theme. In Walkabout, an Australian aborigine teenager helps two white kids to survive in the outback, and when the girl rejects his affections (in the mildest way possible) he loses the will to live and fades into death as mild as a lamb. I guess he had to pay the ultimate price, as it seems most magical black people do, in his case for having any desires of his own.
I assume that the magical black person also includes the black sidekick, long a staple of cartoons and these days best played by Eddie Murphy is Mulan and Shrek. The main difference between the other MBPs is that he grumbles a lot while saving the white person's life. Also don't forget Ernie Hudson in The Basketball Diaries, who locked up Jim Carroll and forcibly detoxed him. I'm sure we could play this game all day, there are plenty of movies with this trope.
The AV Club article suggests that Hollywood writers don't know anything about black people so instead of giving them rich characters and backstories they have no backstory and magical powers. I think there is one other factor at play. Interactions between blacks and whites are on occasions complex and rife with subtext, and so movies do anything they can to make them simpler, whether it's making the characters adoptive brothers (Money Train and that movie where Mark Wahlberg and the other three guys avenge their foster mother), making the black character a fantasy element or 'not real' (What Dreams May Come springs to mind) or making the black character unwaveringly devoted to the white character for some reason (Silver Streak). That way audiences don't have to worry about which side of town who is from and what's going on back at the black character's house, thus uncluttering and unfettering the imagination. Who would want to think about Bagger Vance's family life in 1920s Georgia? That would be depressing.