It's the "one-drop" rule. If you're a little bit black, you're black. A lot of black people in the USA (and in the UK probably) have significant white parentage. I think this is stupid, it doesn't really matter whether you're black or white. What's wrong with just saying "One of my parents has dark skin, my other parent has light skin"? But I guess people are still to some extent culturally segregated by the colour of their skin so they feel a need to pick one of them.