It's not like humans randomly became black or white, skin color is a result of thousands of years of evolution because of our surroundings. If you're going to apply the real world here, then you have to admit that black skinned dwarves make no sense from a biological point of view, since as we can see in the real world things tend to get paler and paler the less UV they're exposed to.
I don't know enough about Tolkien elves to comment on there being no black elves though