Immigration?

Race mixing?

You do know that much of this Hobbit/LOTR fantasy is rooted in European medieval history, right?

And do you also recognize that there were brown people present in Europe during that period in history?

There.

That's the explanation.

Also.. it's a made up world. If they want to have brown people here, they can have that. If they want a Black elf, then that's okay, too. Why not? Explain why not. Elves, Hobbits, Dwarves, and Humans are all individual species, right? Within each species, phenotypical variations occur, right?

There's another explanation.