those SA Chinese who're claiming to be 'black' - are doing it because of racist legislation. If they're born in SA, then they're South African Chinese or Chinese South Africans (ie: of 'Asian ancestry). Thats all.
It doesnt matter whose ancestors are what - once you have two or more generations of people born into a country, then to be whining about where long distant ancestors came from, is to be ending up with your head up your semantic gene-obsessed ass.
Ditto to be trying to raise arguments about whose long dead ancestors humped what goat, or stole whose daughters or sold tribes into slavery - is again, to be bordering on the absurd.
All thats relevant is you, your parents, and their parents, pretty much. Anything else is dead history, and no ones fault - given that neither you, your parents, or probably your grandparents as well, 'chose' where they wanted to be born. Go dig up long dead corpses and argue with them rather.
Where you are born, determines what your 'nationality' is. Its all just luck of the draw, a trick of sperm and egg (assuming some places are 'better' than others, to have been born in.)
Idiocy, is trying to make events of 100-200 years ago and longer, seem like valid arguments now to people who werent there at the time, and who have and had nothing whatsoever to do with the far distant past events.
Deal with whats here in reality, the current situation and its resolution - not trying to assign blame over what primitive ancestors happened to get up to.