The civil rights movement of the 1960's have left many people with the belief that the slave trade was exclusively a European/USA phenomenon and only evil white people were to blame for it. This is a simplicistic scenario that hardly reflects the facts.
Thousands of records of transactions are available on a CDROM prepared by Harvard University and several comprehensive books have been published recently on the origins of modern slavery (namely, Hugh Thomas' The Slave Trade and Robin Blackburn's The Making Of New World Slavery) that shed new light on centuries of slave trading.
What these records show is that the modern slave trade flourished in the early middle ages, as early as 869, especially between Muslim traders and western African kingdoms. For moralists, the most important aspect of that trade should be that Muslims were selling goods to the African kingdoms and the African kingdoms were paying with their own people. In most instances, no violence was necessary to obtain those slaves. Contrary to legends and novels and Hollywood movies, the white traders did not need to savagely kill entire tribes in order to exact their tribute in slaves. All they needed to do is bring goods that appealed to the kings of those tribes. The kings would gladly sell their own kins.
This explains why slavery became "black". Ancient slavery, e.g. under the Roman empire, would not discriminate: slaves were both white and black (so were Emperors and Popes). In the middle ages, all European countries outlawed slavery (of course, they retained countless "civilized" ways to enslave their citizens, but that's another story), whereas the African kingdoms happily continued in their trade. Therefore, only colored people could be slaves, and that is how the stereotype for African-American slavery was born. It was not based on an ancestral hatred of blacks by whites, but simply on the fact that blacks were the only ones selling slaves, and they were selling their own kins.