The best black novel to appear in America since “Beloved” has just been written by a white man. (Or at least so says this honky critic.) With “The Time of Our Singing,” Richard Powers has fulfilled Martin Luther King’s dream of a nation in which authors “will not be judged by the color of their skin but by the content of their” books. Or he’s added another chapter to the nightmare of black experience in which everything of value – including their themes, their struggles, and their history – is appropriated by whites.