Wow. That's pretty strong stuff. Are you sure about this? I don't think too many blacks are going to thank whites for slavery, in fact affirmative action tells me that we're still paying a price for it.
Don't forget Dingaan, the Zulu chief who visited and was entertained by Queen Victoria after having lost the Zulu War, but during which he had enjoyed an extraordinary victory over British troops in a period during which they were lead by an upper class incompetent. Afterwards she wrote to him saying that over 300 wives were to much. He replied that she was right, so he'd killed 300 of them.
Let's move on. Let's read some of the speeches by Lord Salisbury when he was moving to abolish child labour in Britain in the mid 1800's. He spoke of 6 year old girls and boys working in mines for 6 days a week, in near total darkness, amid rats who stole their bread, and being shackled all day long to coal trucks to drag them along on all-fours all day long. This continued until adulthood.
Now read something from the pro-slavery lobby. They pointed out that the birth rate of slave females was quite low - about three each (which is hardly an example of sexual exploitation), that most slaves had their own patch of garden in which to grow vegetables and keep chickens (unheared of in working-class Britain), and that they almost all originated as victims of tribal conflicts and as such would have been killed out-of-hand had they not been sold as slaves. Now, look at their descendants, who after having triggered an absurd Civil War and the biggest advancement programme in human history, still find more of themselves in jail than in University. If I were a white American then I would feel nothing. Those who were really damaged long-term by the import of slaves were the whites themselves.