The Collaborative International Dictionary of English v.0.48:

Dark \Dark\ (d[aum]rk), a. [OE. dark, derk, deork, AS. dearc,
   deorc; cf. Gael. & Ir. dorch, dorcha, dark, black, dusky.]
   1. Destitute, or partially destitute, of light; not
      receiving, reflecting, or radiating light; wholly or
      partially black, or of some deep shade of color; not
      light-colored; as, a dark room; a dark day; dark cloth;
      dark paint; a dark complexion.
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            O dark, dark, dark, amid the blaze of noon,
            Irrecoverably dark, total eclipse
            Without all hope of day!              --Milton.
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            In the dark and silent grave.         --Sir W.
                                                  Raleigh.
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   2. Not clear to the understanding; not easily seen through;
      obscure; mysterious; hidden.
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            The dark problems of existence.       --Shairp.
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            What may seem dark at the first, will afterward be
            found more plain.                     --Hooker.
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            What's your dark meaning, mouse, of this light word?
                                                  --Shak.
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   3. Destitute of knowledge and culture; in moral or
      intellectual darkness; unrefined; ignorant.
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            The age wherein he lived was dark, but he
            Could not want light who taught the world to see.
                                                  --Denhan.
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            The tenth century used to be reckoned by medi[ae]val
            historians as the darkest part of this intellectual
            night.                                --Hallam.
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   4. Evincing black or foul traits of character; vile; wicked;
      atrocious; as, a dark villain; a dark deed.
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            Left him at large to his own dark designs. --Milton.
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   5. Foreboding evil; gloomy; jealous; suspicious.
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            More dark and dark our woes.          --Shak.
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            A deep melancholy took possesion of him, and gave a
            dark tinge to all his views of human nature.
                                                  --Macaulay.
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            There is, in every true woman-s heart, a spark of
            heavenly fire, which beams and blazes in the dark
            hour of adversity.                    --W. Irving.
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   6. Deprived of sight; blind. [Obs.]
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            He was, I think, at this time quite dark, and so had
            been for some years.                  --Evelyn.
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   Note: Dark is sometimes used to qualify another adjective;
         as, dark blue, dark green, and sometimes it forms the
         first part of a compound; as, dark-haired, dark-eyed,
         dark-colored, dark-seated, dark-working.
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   A dark horse, in racing or politics, a horse or a candidate
      whose chances of success are not known, and whose
      capabilities have not been made the subject of general
      comment or of wagers. [Colloq.]

   Dark house, Dark room, a house or room in which madmen
      were confined. [Obs.] --Shak.

   Dark lantern. See Lantern. -- The

   Dark Ages, a period of stagnation and obscurity in
      literature and art, lasting, according to Hallam, nearly
      1000 years, from about 500 to about 1500 A. D.. See
      Middle Ages, under Middle.

   The Dark and Bloody Ground, a phrase applied to the State
      of Kentucky, and said to be the significance of its name,
      in allusion to the frequent wars that were waged there
      between Indians.

   The dark day, a day (May 19, 1780) when a remarkable and
      unexplained darkness extended over all New England.

   To keep dark, to reveal nothing. [Low]
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WordNet (r) 3.0 (2006):

Dark Ages
    n 1: the period of history between classical antiquity and the
         Italian Renaissance [syn: Middle Ages, Dark Ages]