The Collaborative International Dictionary of English v.0.48:

Blackmail \Black"mail`\, n. [Black + mail a piece of money.]
   1. A certain rate of money, corn, cattle, or other thing,
      anciently paid, in the north of England and south of
      Scotland, to certain men who were allied to robbers, or
      moss troopers, to be by them protected from pillage. --Sir
      W. Scott.
      [1913 Webster]

   2. Payment of money exacted by means of intimidation; also,
      extortion of money from a person by threats of public
      accusation, exposure, or censure.
      [1913 Webster]

   3. (Eng. Law) Black rent, or rent paid in corn, flesh, or the
      lowest coin, a opposed to "white rent", which paid in
      silver.
      [1913 Webster]

   To levy blackmail, to extort money by threats, as of injury
      to one's reputation.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blackmail \Black"mail`\, v. t. [imp. & p. p. Blackmailed; p.
   pr. & vb. n. Blackmailing.]
   To extort money from by exciting fears of injury other than
   bodily harm, as injury to reputation, distress of mind, etc.;
   as, to blackmail a merchant by threatening to expose an
   alleged fraud. [U. S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

blackmail
    n 1: extortion of money by threats to divulge discrediting
         information
    v 1: exert pressure on someone through threats [syn:
         blackmail, blackjack, pressure]
    2: obtain through threats

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "blackmail":
   account, allowance, ask, ask for, assessment, badger, badger game,
   bill, blood money, bloodsucking, boodle, booty, call, call for,
   challenge, claim, clamor for, coerce, compel, contribution,
   cry for, demand, demand for, draft, drain, duty, emolument, exact,
   exaction, extort, extortion, extortionate demand, fee, footing,
   force, force from, graft, haul, heavy demand, hot goods,
   hush money, impose, imposition, impost, indent, initiation fee,
   insistent demand, issue an ultimatum, levy, levy blackmail, loot,
   make, make a demand, mileage, nonnegotiable demand, notice, order,
   order up, perks, perquisite, pickings, place an order, plunder,
   pork barrel, prize, protection racket, pry loose from, public till,
   public trough, put in requisition, ransom, reckoning, rend,
   rend from, require, requirement, requisition, retainer,
   retaining fee, rip, rip from, rush, rush order, scot, screw,
   shake down, shakedown, snatch from, spoil, spoils,
   spoils of office, squeeze, stealings, stipend, stolen goods, swag,
   take, tax, taxing, tear from, till, tribute, ultimatum, vampirism,
   warn, warning, wrench, wrench from, wrest, wring, wring from