Source : Webster's Revised Unabridged Dictionary (1913)

Blackfish \Black"fish\, n.
   1. (Zo["o]l.) A small kind of whale, of the genus
      {Globicephalus}, of several species. The most common is
      {G. melas}. Also sometimes applied to other whales of
      larger size.

   2. (Zo["o]l.) The tautog of New England ({Tautoga}).

   3. (Zo["o]l.) The black sea bass ({Centropristis atrarius})
      of the Atlantic coast. It is excellent food fish; --
      locally called also {black Harry}.

   4. (Zo["o]l.) A fish of southern Europe ({Centrolophus
      pompilus}) of the Mackerel family.

   5. (Zo["o]l.) The female salmon in the spawning season.

   Note: The name is locally applied to other fishes.

Source : WordNet®

blackfish
     n 1: large dark-colored food fish of the Atlantic coast of North
          America [syn: {tautog}, {Tautoga onitis}]
     2: female salmon that has recently spawned
     3: small dark-colored whale of the Atlantic coast of the United
        States; the largest male acts as pilot or leader for the
        school [syn: {pilot whale}, {black whale}, {common
        blackfish}, {Globicephala melaena}]
     [also: {blackfishes} (pl)]