Offtopic, but not being from the USA I have real trouble understanding the concept of "blackness" in that country. What does it mean, exactly, to be black? Is

Offtopic, but not being from the USA I have real trouble understanding the concept of "blackness" in that country. What does it mean, exactly, to be black? Is it a genetic, a phenotypic or a cultural thing? How is it determined whether mixed-race people are black? And what about trans-racially adopted children? Does a black kid adopted by a white family become white? Does a white kid adopted by a black family become black? If two mixed-race siblings have different tone of skin, can one of them be black and the other white? I cannot manage to answer all these questions simultaneously without blatant contradictions, unless I assume (apparently, wrongly) that "being black or not" is not a partition of the set of all people. In my country, in europe, it would seem that being black is not an identity but merely a physical characteristic like being blonde. I have never raised the issue explicitly, but I guess my black colleagues at work would not appreciate being classified as a different identity just for their skin color.