Ages ago, humans descended from Negroid, Caucasoid, and Mongoloid stock. Those were the first three "races". "Black" is shorthand for "dark-skinned person who has more visible negroid features than those of the other three original races", that today has very impolite ramifications because in more recent history, one group of people decided they were superior to another group of people and did some really bad things. Now, everything's all messed up.
There's not really a polite shorthand for "that guy with brown skin". We're so sensitive to those bad things that one group of people did in the past that it's considered rude to even notice a guy's skin color, even in a neutral manner. We're supposed to pretend we don't notice. You can use every descriptor under the sun to talk about someone else, but the second you mention he's a "black" guy, you get dirty looks for admitting that you notice skin color.
So someone might think that maybe we refer to nationality instead of race to avoid being offensive by saying, "African American", but a visibly "black" person of Indonesian descent who's a citizen of France is neither African nor American.
You can point and say, "Look at the blond girl", and that's okay, but "Look at the black girl" earns you dirty looks, all because this one bad group of people did some bad things in the past and now nobody can admit that anybody has different skin color ever again.
Hey, thank you for your comment. Why do we need to emphasize the color of a person that much as if tho there is no other way to tell people that we are all the same humans? :)
It's just another adjective.
If there's a black guy walking next to a white guy and I want to tell you something about one of those two guys, why should people be fumbling for words when there's a perfectly easy descriptor to use?
"Uh...the one who's about one inch taller. The one with brown eyes and slightly shorter hair...what, you can't see his eyes? Uh...okay...the guy with the more athletic back...oh fuck it. The black guy."
Race exists. It's one more thing that makes us different. Some of us are taller than others. Some of us are fatter than others. Some of us are men, some are women. Some are old, some are little kids. We have different color hair. Big noses, small noses, big ears, small ears.
I can say, "Look at the short, fat, bald guy with big ears", and we're cool. But if I say, "Look at that black guy over there", the room gets quiet. It's stupid. People have different color skin, just like they have different color hair.