The way races are defined is racist to begin with - that is the point - and the way it is being used to label people is explicitly used to put people down. Obama is "black" and not "white" solely because for a lot of people it is very important to label him black to be able to dismiss him. If people were not racist, nobody would e.g. care if people referred to him as white. Try referring to him even as mixed race, or half-white, and see the racists quickly come crawling out of the woodwork to make sure it is made clear how black he is.