Source : Webster's Revised Unabridged Dictionary (1913)

Blackmailer \Black"mail`er\, n.
   One who extorts, or endeavors to extort, money, by black
   mailing.

Source : WordNet®

blackmailer
     n : a criminal who extorts money from someone by threatening to
         expose embarrassing information about them [syn: {extortioner},
          {extortionist}]