There are several words that can be used as synonyms for the word "darkener". The most commonly used word is "dimmer", which refers to a device that reduces the amount of light emitted by a bulb or lamp. Other synonyms include "shade", which is used to block out light, "tint", which refers to a color that is added to darken something, and "blinder", which is similar to a shade but is typically used to block out light from a specific area or direction. Other synonyms for "darkener" include "eclipse" and "obscure". All of these words can be used to describe ways of reducing or blocking out light to create a darker environment.