The melting pot and ignorance of the world certainly have a lot to do with it, but I also think it goes back a lot further than that. I think it originates with the adoption of slavery by Monotheistic cultures. The teachings of Christianity and Islam, with their emphasis on individual moral responsibility, made slavery incompatible with religious doctrine. In the pagan world, a slave was a prisoner, someone paying the price for committing a crime, or losing a war. The slave was not inferior, or subhuman, he might have been royalty or an aristocrat prior to his enslavement. In fact, highly talented slaves were recognized as valuable assets, they could run your farm, or design your buildings, or manage your business. Many were enslaved for their math and literacy skills, becoming teachers and accountants. Many were employed as scribes, physicians, counselors and consultants. They were slaves (and their children) because they had lost a war or committed a crime, not because they were subhuman creatures. Slavery was something that happened to them, not something they were. Their children automatically were slaves too, a concept natural to tribe and family oriented pagan cultures of the ancient world, but antithetical to the more individual oriented societies of the medieval world.

But Christianity and Islam taught all humans had rights and dignity in the eyes of god, and it was wrong to treat them as property. In order to employ other humans as beasts of burden, or factory machines on an industrial scale, and still profess Christian values it was necessary to dehumanize them. They had to be seen as deserving slavery because they weren't real people. And race was a way of easily visualizing and identifying suitable candidates. In the writings of antiquity, you rarely hear the race of a person mentioned, it simply didn't matter; it was ethnic, class, tribal and family connections that were important. If a Roman had a Greek slave, he didn't think all Greeks deserved to be slaves, or that Romans shouldn't be enslaved at all.

The slave markets of the ancient world were fully integrated, all races, nations and religions contributed to them. It was religion that championed individual virtue and sin that made them segregated by race. Its OK to enslave blacks, just look at them. You can SEE they are slaves. They deserve it. Its in their blood.

What I still can't figure out is how slavery changed in the New World. The Spanish civilization of Latin America employed slaves just as brutally as the Anglos in the North, but when it was finally outlawed those societies accepted it. There was still social racism, but not the sort of malice and hatred shown to the Negro in North America. Why this should be is not clear.