Source : Webster's Revised Unabridged Dictionary (1913)

Blacken \Black"en\, v. i.
   To grow black or dark.

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.

            While the long funerals blacken all the way. --Pope.

   2. To make dark; to darken; to cloud. ``Blackened the whole
      heavens.'' --South.

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

   Syn: To denigrate; defame; vilify; slander; calumniate;
        traduce; malign; asperse.

Source : WordNet®

blacken
     v 1: make or become black; "The smoke blackened the ceiling";
          "The ceiling blackened" [syn: {melanize}, {melanise}, {nigrify},
           {black}] [ant: {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}, {scorch}]