Black Hand Captain is the generic named version of Captain Ioushuwa of the Black Hand stone ship.
Black Hand Captain can be found at the following locations:
He/she shares a spawn point with Captain Ioushuwa, always located on the bow of the ship, overlooking at valley.
It would be best suggested that you take care of all the thralls on the entire upper deck anywhere near the Captain before deciding to take him on. The Captain is a very strong fighter to add to your thrall army.
Black Hand Captain at The Black Galleon