The Collaborative International Dictionary of English v.0.48:

Black-letter \Black"-let`ter\, a.
   1. Written or printed in black letter; as, a black-letter
      manuscript or book.
      [1913 Webster]

   2. Given to the study of books in black letter; that is, of
      old books; out of date.
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            Kemble, a black-letter man!           --J. Boaden.
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   3. Of or pertaining to the days in the calendar not marked
      with red letters as saints' days; -- compare red-letter.
      Hence: Unlucky; inauspicious.
      [1913 Webster]