On the subject of race. Why is there a tacit assumption that all persons are white unless identified as some different race? Example: Maybe a guy is lost from his group at a big convention or something and he tells someone that he is looking for "these three guys... one of them is black, and one of them has a big nose ring?" Like black-ness is an unusual trait to be used to pick somebody out of a crowd or a police line up, like a scar or a tattoo. I hope this made at least some sense.

I agree with Richard. But there is also another sense in which racism leads people to underestimate the number of 'whites'. I am thinking here of the practice of counting somebody as unequivocally 'black' if their ancestry is half European and half African, or even 80% European and 20% African. If skin colour were not considered to be socially and politically significant, this would make no sense at all.