Source : Webster's Revised Unabridged Dictionary (1913)

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®

blackening
     n : changing to a darker color [syn: {darkening}]