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.
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   2. (Zool.) The tautog of New England (Tautoga).
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   3. (Zool.) The black sea bass (Centropristis atrarius) of
      the Atlantic coast. It is excellent food fish; -- locally
      called also black Harry.
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   4. (Zool.) A fish of southern Europe (Centrolophus
      pompilus) of the Mackerel family.
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   5. (Zool.) The female salmon in the spawning season.
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   Note: The name is locally applied to other fishes.
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