Well, Nate Silver lays down a bomb of his own:
I suspect that a lot of the problem, however, is that as Congressional Districts have become more and more gerrymandered, leading to the creation of more and more majority-minority districts following the 1980 and 1990 censuses, the black political apparatus has become more and more 'ghettoized'. Black candidates have not had to develop a message that appeals to white voters, because most of them don't have very many white voters in their districts (about half the nation's African-American population is limited to the 60 blackest Congressional Districts). Nor do they have very many conservative voters in their districts, and so they have not had to develop a message that appeals to conservatives, even though the black population itself is far more diverse in its political views than is generally acknowledged.