Source : Webster's Revised Unabridged Dictionary (1913)

Darkening \Dark"en*ing\, n.
   Twilight; gloaming. [Prov. Eng. & Scot.] --Wright.

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.

            They [locusts] covered the face of the whole earth,
            so that the land was darkened.        --Ex. x. 15.

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

   2. To render dim; to deprive of vision.

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

   3. To cloud, obscure, or perplex; to render less clear or
      intelligible.

            Such was his wisdom that his confidence did seldom
            darkenhis foresight.                  --Bacon.

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

   4. To cast a gloom upon.

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

   5. To make foul; to sully; to tarnish.

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

Source : WordNet®

darkening
     adj 1: becoming dark or darker as from waning light or clouding
            over; "the darkening sky"
     2: characterized by hopelessness; filled with gloom; "gloomy at
        the thought of what he had to face"; "gloomy predictions";
        "a gloomy silence"; "took a grim view of the economy";
        "the darkening mood" [syn: {gloomy}, {grim}]
     n : changing to a darker color [syn: {blackening}]