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One of the more important reasons for the shift from white and black slaves stems of a US based British colony ran mostly by slaves in which the slaves, both white and black banded together for a slave rebellion in an attempt to escape to Spanish Florida where free colonies already existed. They killed a about 25 free whites in their attempt who were outnumbered enormously by slaves in the area at the time. They also burned several buildings and amassed a decent sized following but the rebellion was put down.

In 1740, 2 years after that Stono Rebellion [wikipedia.org] the South Carolina legislature (mostly a corporate board because of the colony situation) passed the Negro Act of 1740 in attempts to control the slaves. It provided protections against harsh working conditions and so on that would create a rebellious situation in the first place but was hard to enforce because a slave couldn't testify against a free man. One of the more influential parts of the Negro act was that it regulated manumissions which is more or less a fancy term for a slave owner granting freedom to their slaves. One of these regulations came to be a divide and conquer strategy in which 1 white slave to every 10 black slaves were required but black slave could never be anything more then a slave where the whites could regain their full citizenship.

This provided a situation where the white slaves would (were encouraged to) report suspected rebellion plots in hopes of gaining their freedom and stopped the entire slave groups from banding together again. This also led to the downfall of white slaves as other restrictions such as importation of new slaves were discouraged/banned and populations were breed from existing stock. A big issue here is corruption of blood, the blacks because of the manumissions laws would always be slaves, including their children where the children of the whites would/could be free people woth full citizenship rights (*another incentive to not rebel and report conspiracies).

This also created the concept of classes among the slaves in which the black slaves were at the bottom by default. This had to do with white slaves appearing smarter because they could already speak the language and mostly read and/or write. Once white slaves fell to the side, the class differences sort of remained which was part of the prejudices throughout early America. Although with the end of the civil war, freed slaves being dumped onto the populations and taking white jobs, and the north mandating the whites succeed power to the freed slaves made the system of racism far more worse then what this was about before then, but slaves weren't really treated with respect either.

The history of racism in the US is deeply tied to slavery and perhap unique to the US because a lot of the laws like the Negro act wasn't enacted in other countries. Combine that with slaves in other countries either finding support from manumissions and former slaves already living and integrating in the other areas or they simply wanted to go home, were in the US, they attempted to create a local stock in which they were already home so to speak and you can see some issues directly connected to slavery that fueled the hate and resentment on both sides.