San Francisco's irrepressible former mayor, Willie Brown, was walking along one of the city's streets when he happened to run into another former city official that he knew, James McCray. McCray's greeting to him was "You're 10." "What are you talking about?" Willie Brown asked. McCray replied: "I just walked from Civic Center to Third Street and you're only the 10th black person I've seen." That is hardly surprising. The black population of San Francisco is less than half of what it was in 1970, and it fell another 19 percent in the past decade. A few years ago,...