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.
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            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:

Darkening \Dark"en*ing\, n.
   Twilight; gloaming. [Prov. Eng. & Scot.] --Wright.
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WordNet (r) 3.0 (2006):

darkening
    adj 1: becoming dark or darker as from waning light or clouding
           over; "the darkening sky"
    n 1: changing to a darker color [syn: blackening, darkening]