As DC gets whiter, so do its mayors. (DC’s black population is at its lowest level since the ’60s.) Here’s Marion Barry. Barry, who is clearly black, was succeeded by Anthony Williams, who was succeeded by Adrian Fenty. Fenty will either win re-election or be succeeded by Vince Gray, following the election (i.e. the Democratic Primary) in a few days. Based on the pictures, you may just have to take my word for it that Fenty and Gray are black.
It appears that being a black democrat (sorry for the redundancy) is a prerequisite for being mayor. But it also appears that as the city whitens, so does the skin tone of the mayoral candidates – especially in this election.
This election is particularly interesting, since Fenty is running with much stronger support from whites (though it has slipped significantly, as Wikipedia says, "His ratings among African American voters dropped to 29 percent from 68 percent, while his support among whites dropped to 57 percent from 78 percent."). Gray is less popular among white voters (white voters in DC = uber-SWPLs) – after all, Gray almost killed streetcars and white people love streetcars.
Anyway, I’m not sure if the trend of having ever-whiter mayors can continue, since I’m not sure they can get any whiter and still pass as black.