For black pop music, this is an exhilarating, unsettling time. Never before have so many black performers been so successful in the world of mainstream pop. Not only have such sure bets as Lionel Richie and Whitney Houston scored big, but there have also been breakthroughs by such wild cards as Run-D.M.C., Sade and Anita Baker – acts that would have had difficulty cracking even the black Top...