The Collaborative International Dictionary of English v.0.48:

Denigrate \Den"i*grate\, v. t. [L. denigrare; de- + nigrare to
   blacken, niger black.]
   1. To blacken thoroughly; to make very black. --Boyle.
      [1913 Webster]

   2. Fig.: To blacken or sully; to defame. [R.]
      [1913 Webster]

            To denigrate the memory of Voltaire.  --Morley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

denigrate
    v 1: cause to seem less serious; play down; "Don't belittle his
         influence" [syn: minimize, belittle, denigrate,
         derogate]
    2: charge falsely or with malicious intent; attack the good name
       and reputation of someone; "The journalists have defamed me!"
       "The article in the paper sullied my reputation" [syn:
       defame, slander, smirch, asperse, denigrate,
       calumniate, smear, sully, besmirch]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "denigrate":
   asperse, bedarken, besmirch, bespatter, black, blacken, blackwash,
   blot, blotch, call names, calumniate, charcoal, cork, darken,
   defame, defile, deprecate, depreciate, despise, dinge, discommend,
   disdain, disparage, dispraise, disvalue, ebonize,
   engage in personalities, heap dirt upon, hold in contempt, ink,
   libel, melanize, muckrake, murk, nigrify, oversmoke, put down,
   revile, ridicule, scandalize, shade, shadow, slander, smear,
   smirch, smoke, smudge, smut, smutch, soil, soot, stigmatize, sully,
   tarnish, tear down, throw mud at, traduce, vilify