"When black men shoot at other black men in the city, they're practicing their own version of Stand Your Ground self-defense. Why does this law never apply to them?"
Sometimes it does apply to them, but often it doesn't -- especially when they're committing a dozen different felonies when the firearm is used. Obviously, urban violence is closely tied to the drug trade, and simply possessing of a weapon during a drug transaction will get anyone (regardless of their color) a one-way ticket to jail. Additionally, most large cities have laws against conceal carry, ensuring that only criminals have firearms.