Magic Negro?

I'm not very sophisticated, but I can't figure out what these people are talking about. I'm sitting here stunned when I read this, and it's from the LA Times.

Like a comic-book superhero, Obama is there to help, out of the sheer goodness of a heart we need not know or understand. For as with all Magic Negroes, the less real he seems, the more desirable he becomes. If he were real, white America couldn't project all its fantasies of curative black benevolence on him.

A Magic Negro? Jeez!