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]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "darkening":
   blackening, blocking the light, burial, burying, clouding,
   concealedness, concealment, covering, covering up, covertness,
   deception, denigration, dimming, eclipsing, extinguishment,
   hiddenness, hiding, interment, invisibility, masking, melanization,
   mystification, nigrification, obfuscation, obnubilation,
   obscuration, obscurement, obumbration, occultation, occulting,
   overcast, overclouding, overshading, overshadowing, overshadowment,
   putting away, screening, secrecy, secretion, shading, shadowing,
   smirch, smirching, smudge, smudging, smut, smutch, smutching,
   subterfuge, uncommunicativeness