I remember reading a story once where we had all interbred such that there were no longer any blacks, whites, Asians, etc. We were all 'brown'. Somehow they 'found' 5 children that were purebred 'races', and I don't remember much else.
I'll just quote the text of it:
It involves a 20th century white boy accidentally scratching off the skin of his friend, showing that it's all brown underneath, and that there's some vast conspiracy that everyone but him is involved in. I want to say that his friend still helps him, but the majority of it is him corresponding with a penpal through coded letters (they do something like having the message be in every third letter) to escape and find more people like him.
Of course, the reveal isn't that the people around him aren't human, but that
The human race has become a homogenous brown due to the mixing of races here in the 24th century, and the protagonist, his pen pal, and a few others are genetic throwbacks to earlier races being preserved in a zoolike "natural environment".