I don't really know the answer but my guess is that the fiction you are referring to is probably set in a future time where we are all a big brotherhood of man who would elect a president based on character and not skin colour. I think Martin Luther King thought along these lines too. Electing a black president would be a big deal - just check out the recent Democratic nomination battle and all the racial divisions it has brought to the surface. On a personal note, I think it is a sad world we live in where the colour of your skin is even an issue. Maybe all those fictional black presidents have been created by people who have hope for the future and maybe we need more people like them.