I worked at a lot of fish shops and generally black-fin shark has turned out to be columbian-sharks, columbian cat-sharks, whatever you want to call them.
I have kept a few of them. Brackish water towards marine in maturity is best for them, as someone had said before on this thread. If not, expect fungal infections. Also they get pretty big (12"+) and are voracious eaters in my experience.
Constantly moving, never dull and can and will annoy the crap out of more layed-back fish.
Kept mine in a brackish with a puffer, 2 archers, 2 argentus, and an eel.
I HAVE also heard of the Bala Shark being called a black-fin shark. If so, this changes the game entirely.
There are lots of "sharks" out there that LFS's get confused on the nametag.
i bought those from wal-mart once..then found out they needed brackish and brang them bach and told them to add salt..but they dident believe me..hey y yould they believe a kid anyways.