blacken
To make something turn black or very dark.
To blacken means to make something black or very dark, usually by burning, staining, or covering it with soot or dirt. When toast burns in the toaster, it blackens. When a cook blackens fish in a very hot skillet with special spices, the outside gets a dark, flavorful crust while the inside stays tender.
Blacken can also mean to damage someone's reputation by saying harmful things about them. If someone spreads false rumors about a classmate, they're trying to blacken that person's name or reputation. This usage comes from the idea of staining something pure (like a white cloth) with something dark and dirty. When a politician's opponents try to blacken her character, they're attempting to make others think badly of her.
The sky can blacken before a storm, filling with dark clouds. A bruise blackens as it heals, changing from red to purple to dark blue-black. In each case, something becomes darker than it was before, whether literally or figuratively.