Regardless of Limbaugh's unintentional irony with his "Barack the Magic Negro"(his somewhat thick-headed defense being that "liberals did it first", failing or refusing to understand the actual intention, significance, and meaning of the term), the magical negro phenomenon is fantastically explained here. Basically, magical negro-ism is a tendency in certain narratives (movies, especially) to have a near-perfect black character, whose job it is to redeem the white main character. The problem with this, as the linked article points out, is that it deprives the black character of an inner life and their own narrative arc and agency.
This post is apropos of nothing, it was just mentioned in the context of this review of the movie Australia. Interestingly enough, Daktari has a similar post on sexism today. (Well, vaguely similar -- it's on sexism, but it's similar in that she's discussing the subtleties between sexiness, sexism, and pedophilic objectification.)
Anyway. The concept is well illustrated in the commentary's accompanying cartoon:
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