No. They weren't natural born citizens after the civil war either. It was only the second generation after the civil war that became "natural citizens."
All the former slaves were "naturalized" by an act of man called the 14th amendment. Their subsequent children were natural citizens, but the former slaves were not.
If slaves and the children born of slaves were not Natural Born citizens, then the term Natural Born Citizen has nothing whatsoever to do with Natural Law.