My question was to do with how people talk about themselves in South Africa - is it normal to say you are white South African instead of just South African. Because if it is no more normal to do that than in say the UK, there is a clear implication: "don't worry, we are not black!"

If it is normal for a South African to describe themselves in this way, then the enquiry was completely innocent of any racist undertone.

Someone saying they are a black American may or may not have its origin in racist intent, but it would alarm me less, as stated above, because it comes from the oppressed, not the oppressor.