VERB (3)

1. become dark or darker;
- Example: "The sky darkened"

2. tarnish or stain;
- Example: "a scandal that darkened the family's good name"

3. make dark or darker;
- Example: "darken a room"

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.
      [1913 Webster]

            So spake the Sovran Voice; and clouds began
            To darken all the hill.               --Milton.
      [1913 Webster]

   2. To render dim; to deprive of vision.
      [1913 Webster]

            Let their eyes be darkened, that they may not see.
                                                  --Rom. xi. 10.
      [1913 Webster]

   3. To cloud, obscure, or perplex; to render less clear or
      intelligible.
      [1913 Webster]

            Such was his wisdom that his confidence did seldom
            darkenhis foresight.                  --Bacon.
      [1913 Webster]

            Who is this that darkeneth counsel by words without
            knowledge?                            --Job.
                                                  xxxviii. 2.
      [1913 Webster]

   4. To cast a gloom upon.
      [1913 Webster]

            With these forced thoughts, I prithee, darken not
            The mirth of the feast.               --Shak.
      [1913 Webster]

   5. To make foul; to sully; to tarnish.
      [1913 Webster]

            I must not think there are
            Evils enough to darken all his goodness. --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Darken \Dark"en\, v. i.
   To grow or darker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

darken
    v 1: become dark or darker; "The sky darkened" [ant: lighten,
         lighten up]
    2: tarnish or stain; "a scandal that darkened the family's good
       name"
    3: make dark or darker; "darken a room" [ant: brighten,
       lighten, lighten up]

Moby Thesaurus II by Grady Ward, 1.0:

139 Moby Thesaurus words for "darken":
   attaint, bandage, beat down, becloud, bedarken, bedaub, bedazzle,
   bedim, befog, begloom, bemist, benight, besmear, besmirch, besmoke,
   bestain, black, black out, blacken, blackwash, blanch, blind,
   blind the eyes, blindfold, block the light, blot, blot out, blotch,
   blur, blush, brand, brown, cast a shadow, cast down, change color,
   charcoal, clabber up, cloud, cloud over, cloud up, color, cork,
   crimson, damp, dampen, dampen the spirits, darken over, darkle,
   dash, daub, daze, dazzle, deject, denigrate, depress,
   deprive of sight, dim, dim out, dinge, dirty, discolor, discourage,
   dishearten, dispirit, dusk, ebonize, eclipse, encloud,
   encompass with shadow, enmist, excecate, flush, fog, glare, gloam,
   gloom, glow, gouge, grow dark, grow dim, haze, hoodwink, ink,
   knock down, look black, lower, lower the spirits, make blind,
   mantle, mark, melanize, mist, murk, nigrify, nubilate, obfuscate,
   obnubilate, obscure, obumbrate, occult, occultate, opaque, oppress,
   overcast, overcloud, overshadow, oversmoke, pale, press down,
   redden, sadden, scorch, sear, shade, shadow, singe, sink, slubber,
   slur, smear, smirch, smog, smoke, smudge, smut, smutch, snow-blind,
   soil, somber, soot, strike blind, taint, tarnish, turn color,
   turn pale, turn red, weigh heavy upon, weigh upon, whiten