The Collaborative International Dictionary of English v.0.48:

Black letter \Black" let`ter\
   The old English or Gothic letter, in which the Early English
   manuscripts were written, and the first English books were
   printed. It was conspicuous for its blackness. See Type.
   [1913 Webster]

WordNet (r) 3.0 (2006):

black letter
    n 1: a heavy typeface in use from 15th to 18th centuries [syn:
         Gothic, black letter]