1. To cause something to become blacker in appearance. A noun or pronoun can be used between "blacken" and "up." I want you to crank the heat on the stove to really blacken up the outside of that fillet. The artist blackens up his hand with soot and charcoal before smearing them across his canvas.
2. To wear makeup meant to approximate black skin; to wear blackface. The senator has been under fire after a photo of her from high school shows her blackening up for a Halloween party in 1992. There are a few countries with traditional festivals and ceremonies for which blackening up is not considered a social taboo.
See also: up
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.