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):

blackening
    n 1: changing to a darker color [syn: blackening, darkening]