darken

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To make dark or darker.
• intransitive verb To give a darker hue to.
• intransitive verb To fill with sadness; make gloomy.
• intransitive verb To render vague or uncertain; obscure.
• intransitive verb To tarnish or stain.
• intransitive verb To become dark or darker.

from The Century Dictionary.

• To grow dark or darker.
• To grow less white or clear; assume a darker hue or appearance: as, white paper darkens with age.
• To deprive of light; make dark or darker: as, to darken a room by closing the shutters.
• To obscure or shut out the light of.
• To render less white or clear; impart a darker hue to: as, exposure to the sun darkens the complexion.
• To obscure or cloud the meaning or intelligence of; perplex; render vague or uncertain.
• To render gloomy; sadden.
• To deprive of vision; strike with blindness.
• Hence To deprive of intellectual or spiritual light; sink in darkness or ignorance.
• To sully; make foul; make less bright or lustrous.
• To hide; conceal.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To make dark or black; to deprive of light; to obscure.
• transitive verb To render dim; to deprive of vision.
• transitive verb To cloud, obscure, or perplex; to render less clear or intelligible.
• transitive verb To cast a gloom upon.
• transitive verb To make foul; to sully; to tarnish.
• intransitive verb To grow or darker.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb transitive To make dark or darker by reducing light.
• verb intransitive To become dark or darker (having less light).
• verb transitive To make dark or darker in colour.
• verb intransitive To become dark or darker in colour.
• verb transitive To render gloomy, dark(er) of mood
• verb intransitive To become gloomy, dark of mood
• verb transitive To blind, impair eyesight
• verb intransitive To be blinded, loose clear vision

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb become dark or darker
• verb make dark or darker
• verb tarnish or stain