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]