Ten's of thousands of blacks in THIS country were slave owners, well into the mid 1800's.
The first "legal" slave owner in this country (Virginia colony) was black.
Some of the slaves themselves had been slave owners or slave dealers in Africa, slavery was very common in Africa, and being the owner or being a slave, or being a slave trade worker or slave, was not exclusive of each other.
The man who wrote Amazing Grace had been a slave in Africa, the media only describes him as a Captain trafficking in slaves, which is a great story in itself, but revealing that his years in slavery had an impact on him makes for an even richer tale.