There were certainly some free black men that owned slaves but I would also point out that quite a few of those were actually free black men who "purchased" their wives who happened to be slaves. Unless they went the extra effort to have them legally emancipated, they were owned by their husbands and any children that were born from that union were also considered slaves.
The idea that there were free black business men (planters, etc) who owned slaves is difficult for many to come to terms with, but it is true that they existed. However, most of the free black men who owned slaves actually owned their immediate family members that they were able to purchase out of slavery. There were also some church ministers that were able to purchase some of their members out of slavery.
I believe that the fact is that a large majority of the black slave owners were mulattos (or quadroons or octaroons--one drop of black blood put you in the black category) in Louisiana, where under the French (and carrying over into subsequent decades), mixed race children could inherit their father's property and writing your will to do so was not uncommon.
For instance, sources indicate there were 125 black slave owners in South Carolina, 89 in North Carolina, and 3000 in New Orleans alone.