(Reposted to be rebloggable!)
Any time Tolkien has south/eastern people as antagonists, it’s because they’ve been coerced by guys like Sauron (nor are non-white people the only cultures to be coerced). The association of Eastern/Southern people with Dark Lords is largely an unhappy circumstance of geography rather than some failing of character. Sam has a nice monologue in the Two Towers about how crappy it would be to live in those areas, since that’s where Sauron ran around uncontested for millennia and got to threaten them into fighting. Also, the heroes of Tolkien’s work are usually “white,” but only insofar as their skin is generally light colored. This is likely because the majority of the history covered takes place in the far north of the planet (especially in the 1st Age). All other “racial features” don’t correspond with any real-life ethnic group, except possibly Rohan, who are inspired by ancient Anglo-Saxons. In that case, though, it’s worth pointing out that the people of Rohan are considered inferior by most of the bigger civilizations like Numenor and Gondor.
Regarding “half-trolls,” I don’t know the quote, but nobody in Tolkien’s work describes southern or eastern people as “half-trolls.” There are half-trolls in RoTK, but they’re not referring to people. Sauron’s fighting force is very diverse in that army, including humans, orcs, trolls and others. Apparently Tolkien’s “half-trolls” were inspired by a mythological offspring of a human woman and an incubus. It’s also probably worth pointing out that when Tolkien uses the term “black skin” he only (and frequently) uses it to describe monsters like orcs, never human beings, as he literally means burnt black. He always describes what we would call “black people” as dark brown-skinned.
Personally, I don’t think Tolkien’s work is racist, but the perspective of the narrative is mostly Eurocentric/Ethnocentric because those are the characters who “wrote” it, and even then it’s not actually Europe or a specific real-world ethnicity. What I mean is that the stories are meant to be narrated by in-universe characters from a very narrow cultural perspective (usually Hobbits). There is never any objective narration in any of Tolkien’s work; there is always a biased narrator of events. You might call some of the perspectives racist, but I think it’s more accurate to think of them as medieval people trying to make sense of exotic cultures from Africa and Asia (the dark, mysterious orient in their eyes). This is also why Tolkien’s work doesn’t have much in the way of describing villains like Sauron; they’re the enemies, so the narrators don’t see or understand them! Tolkien was obsessed with philology, and to ignore the implications of that is to miss a big reason he even wrote any books to begin with. Tolkien wasn’t putting together a body of fiction as much as he was collecting “historical” writings to generate the backbone of the languages he created.
It’s also important to keep in mind that none of the races in Middle-Earth are meant to literally correspond with those in the modern world. Hobbits are inspired by the rural English, but modern English people aren’t descended from Hobbits, nor are brown-skinned people from Harad actually ancestors of modern people from North Africa. My point is that modern or historical allegories are rare, if ever, present in Tolkien’s writings. Tolkien hated allegory, and it would be poor critical analysis to search for deliberate real-world comparisons.
It is a complicated topic though, and much of it’s open to interpretation. For a more detailed analysis of racism in Tolkien’s work, I suggest reading this: http://tolkiengateway.net/wiki/Racism_in_Tolkien’s_Works