I don't have an answer... but in case you didn't know, he's probably descended from
http://www.wikitree.com/wiki/Blackwell-485 . Robert Blackwell has a lot of descendants. Wikitree seems only really to have a few of the Blackwell's Island/Long Island families, but there were a chunk in New Jersey and some went down into southern colonies.
http://www.nj.gov/state/archives/chncl004.html shows a Thomas J. Blackwell registering the birth of a slave's child in 1811. I'd imagine he's a close relative if not the father.
