Bratton was also head of the police department in Boston and Los Angeles. Appointing Bratton cost Los Angeles mayor Jim Hahn a second term. Hahn, a white liberal Democrat, had been elected in 2001 over Antonio Villaraigosa by a fragile coalition of blacks and Republicans. When the black police chief Bernard Parks' term was up, Hahn decided to let him go and pick the top available guy in the country, Bratton. This angered blacks, costing Hahn re-election in 2005.
I suspect that it's not going to work like that in New York this time. New York is too important to too many important people to let petty black politics get in the way of law and order. But, we shall see.