The Collaborative International Dictionary of English v.0.48:

Darken \Dark"en\ (d[aum]rk"'n), v. t. [imp. & p. p. Darkened
   (-'nd); p. pr. & vb. n. Darkening (-n*[i^]ng).] [AS.
   deorcian. See Dark, a.]
   1. To make dark or black; to deprive of light; to obscure;
      as, a darkened room.
      [1913 Webster]

            They [locusts] covered the face of the whole earth,
            so that the land was darkened.        --Ex. x. 15.
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            So spake the Sovran Voice; and clouds began
            To darken all the hill.               --Milton.
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   2. To render dim; to deprive of vision.
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            Let their eyes be darkened, that they may not see.
                                                  --Rom. xi. 10.
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   3. To cloud, obscure, or perplex; to render less clear or
      intelligible.
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            Such was his wisdom that his confidence did seldom
            darkenhis foresight.                  --Bacon.
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            Who is this that darkeneth counsel by words without
            knowledge?                            --Job.
                                                  xxxviii. 2.
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   4. To cast a gloom upon.
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            With these forced thoughts, I prithee, darken not
            The mirth of the feast.               --Shak.
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   5. To make foul; to sully; to tarnish.
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            I must not think there are
            Evils enough to darken all his goodness. --Shak.
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The Collaborative International Dictionary of English v.0.48:

darkened \darkened\ adj.
   overtaken by night or darkness.

   Syn: nighted.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

darkened
    adj 1: (of fabrics and paper) grown dark in color over time;
           "the darkened margins of the paper"
    2: become or made dark by lack of light; "a darkened house";
       "the darkened theater"

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "darkened":
   bedazzled, besmirched, blackened, blindfold, blindfolded, dazed,
   dazzled, discolored, excecate, foxed, foxy, hoodwinked, inky,
   murky, obscured, smirched, smoky, snow-blind, snow-blinded, soiled,
   stained, stigmatic, stigmatiferous, stigmatized, tainted,
   tarnished