The Collaborative International Dictionary of English v.0.48:

Blacken \Black"en\, v. i.
   To grow black or dark.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blacken \Black"en\, v. t. [imp. & p. p. Blackened; p. pr. &
   vb. n. Blackening.] [See Black, a., and cf. Black, v.
   t. ]
   1. To make or render black.
      [1913 Webster]

            While the long funerals blacken all the way. --Pope.
      [1913 Webster]

   2. To make dark; to darken; to cloud. "Blackened the whole
      heavens." --South.
      [1913 Webster]

   3. To defame; to sully, as reputation; to make infamous; as,
      vice blackens the character.
      [1913 Webster]

   Syn: To denigrate; defame; vilify; slander; calumniate;
        traduce; malign; asperse.
        [1913 Webster]

WordNet (r) 3.0 (2006):

blacken
    v 1: make or become black; "The smoke blackened the ceiling";
         "The ceiling blackened" [syn: blacken, melanize,
         melanise, nigrify, black] [ant: white, whiten]
    2: burn slightly and superficially so as to affect color; "The
       cook blackened the chicken breast"; "The fire charred the
       ceiling above the mantelpiece"; "the flames scorched the
       ceiling" [syn: char, blacken, sear, scorch]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "blacken":
   abuse, asperse, attaint, bark at, becloud, bedarken, bedaub, bedim,
   begloom, begrime, berate, besmear, besmirch, besmoke, besmutch,
   besoil, bespatter, bestain, betongue, black, black out, blackwash,
   block the light, blot, blot out, blotch, blow upon, blur, brand,
   brown, call names, cast a shadow, cast aspersions on, censure,
   charcoal, cloud, cloud over, cork, darken, darken over, daub,
   defame, defile, denigrate, dim, dim out, dinge, dirty, disapprove,
   discolor, discredit, disparage, ebonize, eclipse, encloud,
   encompass with shadow, engage in personalities, execrate, expose,
   expose to infamy, fulminate against, gibbet, gloom, hang in effigy,
   heap dirt upon, ink, jaw, libel, load with reproaches, malign,
   mark, melanize, muckrake, murk, nigrify, obfuscate, obnubilate,
   obscure, obumbrate, occult, occultate, overcast, overcloud,
   overshadow, oversmoke, pillory, rag, rail at, rate, rave against,
   reprimand, revile, scorch, sear, shade, shadow, singe, slander,
   slubber, slur, smear, smirch, smoke, smouch, smudge, smut, smutch,
   soil, somber, soot, spot, stain, stigmatize, sully, taint, tarnish,
   throw mud at, thunder against, tongue-lash, traduce, vilify,
   vituperate, yell at, yelp at