Source : Webster's Revised Unabridged Dictionary (1913)

Coal-black \Coal"-black`\ (-bl[a^]k`), a.
   As black as coal; jet black; very black. --Dryden.

Source : WordNet®

coal-black
     adj : of the blackest black; similar to the color of jet or coal
           [syn: {jet}, {jet-black}, {pitchy}, {sooty}]