Fifty years after the March on Washington, mystical memories of that seminal moment in the civil-rights era are less likely to focus on movement politics than on the great poetry and great music. The emotional uplift of the monumental march is a universe of time away from today's degrading rap music—filled with the n-word, bitches and "hoes"—that confuses and depresses race relations in America now. The poetry of Aug. 28, 1963, is best on view when Martin Luther King Jr. went off his speech script and started using a musical, chanting reprise—"I have a dream." The transforming insight born of...