The Collaborative International Dictionary of English v.0.48:
Blackfish \Black"fish\, n.
1. (Zool.) A small kind of whale, of the genus
Globicephalus, of several species. The most common is
Globicephalus melas. Also sometimes applied to other
whales of larger size.
2. (Zool.) The tautog of New England (Tautoga).
3. (Zool.) The black sea bass (Centropristis atrarius) of
the Atlantic coast. It is excellent food fish; -- locally
called also black Harry.
4. (Zool.) A fish of southern Europe (Centrolophus
pompilus) of the Mackerel family.
5. (Zool.) The female salmon in the spawning season.
Note: The name is locally applied to other fishes.