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.