What does 'black' mean in America?

This lady - Gabrielle Reece - is black.

It fascinates me that different countries can have such very disparate ways of defining and talking about race. In America, apparently, when it comes to black and white, those are the only two options. If you have ever had a black person in your ancestry, you are, de facto, black (according to the article below, 'Celebs you didn't know were black' - and so much else in the media).

That seems weirdly simplistic to us South Africans, for whom 'black' is itself a broad simplification of a huge variety of tribal and cultural identities. And where 'coloured' means of mixed race, but of a certain, specific established culture and going back centuries, and 'mixed race' is something else entirely, but generally means your mom and dad were any kind of mix, whether black and white, or coloured and black, or indian and black, or white and coloured, or whatever.

Why we humans have to constantly classify each other in this way is another question - but right now, I am fascinated by WHY someone in America who is obviously mostly white, with a little black thrown in, is 'black'. This article (link below) is an example of what I mean - something that seems to me to smack of inherent racism, as if we're supposed to somehow be surprised and amazed that these 'celebs' are not 'white'. And - I've been wanting to ask this for years - why is Obama the "first black president", when he is obviously both black and white? His father is black, his mother is white. Why does the one fact outweigh the other? Why is he never called "the first Hawaiian President"? Or to put it another way, how come, when someone from mostly black ancestry has one white grandparent, the media never calls them 'white'?

Honestly, the American obsession with who is and isn't black strikes me as deeply racist, in an institutionalised, taken-for granted way that's more disturbing to me than the way my own country talks about and conceptualises race. For us, an article like this, singling out people because they 'look white' but happen to have 'black' genes - well, it would be unthinkable. Ludicrous. Offensive. It smacks of the kind of race classification that apartheid made infamous.

Is it just me - or is this article offensive to anyone else? I'd be interested to know.

This is the slideshow that got me pondering on this stuff...
http://www.pressroomvip.com/celebs-you-didnt-know-are-black-4b/26/