They need an explanation for why there are no black people in LOTR? Easy. They took the wrong side in the first wars and are now dead. None of the dark creatures from the first wars survived besides those that were extremely powerful like the Balrog. Any race of human or human-like creature that sided with the dark wouldn't make it. All the human-like creatures that are dark in recent times are creations of the current generation or relatively immediately prior generation of dark lords.