Forgive me if I may offend some people for I am not a rascist person, but I fail to see why calling a person black is such an offence. Being a "white" myself I would not get offended at all at being called a white so and so, even though it may be said at me as a slur. It would be the so and so that would be the most offending me thinks. As a physically defining feature (skin colour) not the same as someones height or weight.
