This comparison doesn't work. First of all, elves and hobbits are different species, while black and white people are not. Secondly, there are no elvish actors. This is the only way the comparison will work. In your scenario, they have cast an actor regardless of race, but altered the characters 'race' anyway. Not to mention Lord of the Rings depends on different and separate 'kingdoms' of human-like races to establish its unique fantasy world.