What does blacken mean?
The definition of blacken
Black”en, v. t.
[imp. & p. p. Blackened; p. pr. & vb. n. Blackening.]
Etym: [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.

Synonyms
— To denigrate; defame; vilify; slander; calumniate; traduce; malign; asperse.

Black”en, v. i.
To grow black or dark.