Source : Webster's Revised Unabridged Dictionary (1913)

Black-letter \Black"-let`ter\, a.
   1. Written or printed in black letter; as, a black-letter
      manuscript or book.

   2. Given to the study of books in black letter; that is, of
      old books; out of date.

            Kemble, a black-letter man!           --J. Boaden.

   3. Of or pertaining to the days in the calendar not marked
      with red letters as saints' days. Hence: Unlucky;
      inauspicious.