Something about the earth in Rohan,it may help you some,please read it as following:
Look to the geography of our world, then look to Middle-Earth. See any differences and similarities, There's quite a few things that are the same. Middle Earth represents the Eurasian continent along with the near East and south. Tolkien did not choose to incorporate black characters(rohan gold) because black people simply are not around in the area in which takes place. It's not racist or anything. Tolkien was just being realistic.
Leastways, not very many, and if so, only once in a blue moon. In a hypothetical situation, three men of darker complexion from the Deep South have traveled North to Gondor. There, they make livings as rich merchants, having established a new sea-going trade route. For the same reasons you do not see a black guy in Julius Caesar, and Macbeth, do you not see them in other works like LOTR. What would black men be doing up in Gondor? In Arnor? There's nothing at those places significant enough to attract a large scale migration. So in short, black men would never be in Gondor. We provide rohan crone for you.
There are only three of them in Rohan. Three out of perhaps 1,000,000 people in Gondor. What are the odds that we just happen to see them, versus some other Gondorian over the course of the LOTR storyline? In a story where believability is prime, black men are not in Gondor, or are too much of a minority to show up for very long. Tolkien could have had a black character with a significant role, but that would be unrealistic considering the geography and numeracy of the other peoples up north.