Doesn't it?

When it comes to US demographics, "black" covers a "natural" cluster of the population / identifiable blob in thingspace. Sure, there are border cases like mixed-race people and recent African immigrants, just like there are edge-cases between bleggs and rubes. "Is person X black or not?" is probably one of the top yes/no questions that would tell you the most about an American (Along with "Did he vote for Obama?", "Is he richer or poorer than the median?", or "Does he live north or south of the Mason-Dixon line?")

Sure, when it comes to world demographics, or Brazilian demographics, it doesn't cut reality at it joints as well.