The Collaborative International Dictionary of English v.0.48:

Coal-black \Coal"-black`\ (-bl[a^]k`), a.
   As black as coal; jet black; very black. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coal-black
    adj 1: of the blackest black; similar to the color of jet or
           coal [syn: coal-black, jet, jet-black, pitchy,
           sooty]