The Collaborative International Dictionary of English v.0.48:

Blackfriar \Black"fri`ar\, Black friar \Black"
fri`ar\(bl[a^]k"fr[imac]`[~e]r). (Eccl.)
   A friar of the Dominican order, so named because wearing the
   black mantle of the Dominicans; -- called also predicant
   and preaching friar; in France, Jacobin. Also, sometimes,
   a Benedictine.

   Syn: Dominican.
        [1913 Webster]

WordNet (r) 3.0 (2006):

Black Friar
    n 1: a Roman Catholic friar wearing the black mantle of the
         Dominican order [syn: Dominican, Black Friar,
         Blackfriar, friar preacher]