Slaves in those days were seen as like cash in the bank today. Some of the blacks that owned black slaves were in fact the mix race offspring of white plantation owners and slave women. Some of these white plantation owners left (in their wills) some of their slaves for their mixed race children as a way of taking care of them.
Another little secret is that MANY IRISH were SLAVES too!
Caught from the streets of England and enslaved for seven years.
Also true in the older British colonies like Barbados, St Kitts and Jamaica. Even today some their descendants live in poverty in parts of rural Jamaica and Barbados, shunned by all, black and white. They call them "ecky becky" in Barbados. Rihanna has some of this ancestry on her father's side.
And BTW that 7 year limit was a disaster because the slave owners wanted to get as much as they could from them before that period ended and so large numbers didn't even live that long. Ironically white slaves were treated worse than the black ones.