Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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