The Collaborative International Dictionary of English v.0.48:

Blue \Blue\ (bl[=u]), a. [Compar. Bluer (bl[=u]"[~e]r);
   superl. Bluest.] [OE. bla, blo, blew, blue, livid, black,
   fr. Icel.bl[=a]r livid; akin to Dan. blaa blue, Sw. bl[*a],
   D. blauw, OHG. bl[=a]o, G. blau; but influenced in form by F.
   bleu, from OHG. bl[=a]o.]
   1. Having the color of the clear sky, or a hue resembling it,
      whether lighter or darker; as, the deep, blue sea; as blue
      as a sapphire; blue violets. "The blue firmament."
      --Milton.
      [1913 Webster]

   2. Pale, without redness or glare, -- said of a flame; hence,
      of the color of burning brimstone, betokening the presence
      of ghosts or devils; as, the candle burns blue; the air
      was blue with oaths.
      [1913 Webster]

   3. Low in spirits; melancholy; as, to feel blue.
      [1913 Webster]

   4. Suited to produce low spirits; gloomy in prospect; as,
      thongs looked blue. [Colloq.]
      [1913 Webster]

   5. Severe or over strict in morals; gloom; as, blue and sour
      religionists; suiting one who is over strict in morals;
      inculcating an impracticable, severe, or gloomy mortality;
      as, blue laws.
      [1913 Webster]

   6. Literary; -- applied to women; -- an abbreviation of
      bluestocking. [Colloq.]
      [1913 Webster]

            The ladies were very blue and well informed.
                                                  --Thackeray.
      [1913 Webster]

   Blue asbestus. See Crocidolite.

   Blue black, of, or having, a very dark blue color, almost
      black.

   Blue blood. See under Blood.

   Blue buck (Zool.), a small South African antelope
      (Cephalophus pygm[ae]us); also applied to a larger
      species ([AE]goceras leucoph[ae]us); the blaubok.

   Blue cod (Zool.), the buffalo cod.

   Blue crab (Zool.), the common edible crab of the Atlantic
      coast of the United States (Callinectes hastatus).

   Blue curls (Bot.), a common plant (Trichostema
      dichotomum), resembling pennyroyal, and hence called also
      bastard pennyroyal.

   Blue devils, apparitions supposed to be seen by persons
      suffering with delirium tremens; hence, very low
      spirits. "Can Gumbo shut the hall door upon blue devils,
      or lay them all in a red sea of claret?" --Thackeray.

   Blue gage. See under Gage, a plum.

   Blue gum, an Australian myrtaceous tree (Eucalyptus
      globulus), of the loftiest proportions, now cultivated in
      tropical and warm temperate regions for its timber, and as
      a protection against malaria. The essential oil is
      beginning to be used in medicine. The timber is very
      useful. See Eucalyptus.

   Blue jack, Blue stone, blue vitriol; sulphate of copper.


   Blue jacket, a man-of war's man; a sailor wearing a naval
      uniform.

   Blue jaundice. See under Jaundice.

   Blue laws, a name first used in the eighteenth century to
      describe certain supposititious laws of extreme rigor
      reported to have been enacted in New Haven; hence, any
      puritanical laws. [U. S.]

   Blue light, a composition which burns with a brilliant blue
      flame; -- used in pyrotechnics and as a night signal at
      sea, and in military operations.

   Blue mantle (Her.), one of the four pursuivants of the
      English college of arms; -- so called from the color of
      his official robes.

   Blue mass, a preparation of mercury from which is formed
      the blue pill. --McElrath.

   Blue mold or Blue mould, the blue fungus (Aspergillus
      glaucus) which grows on cheese. --Brande & C.

   Blue Monday,
      (a) a Monday following a Sunday of dissipation, or itself
          given to dissipation (as the Monday before Lent).
      (b) a Monday considered as depressing because it is a
          workday in contrast to the relaxation of the weekend.


   Blue ointment (Med.), mercurial ointment.

   Blue Peter (British Marine), a blue flag with a white
      square in the center, used as a signal for sailing, to
      recall boats, etc. It is a corruption of blue repeater,
      one of the British signal flags.

   Blue pill. (Med.)
      (a) A pill of prepared mercury, used as an aperient, etc.
      (b) Blue mass.

   Blue ribbon.
      (a) The ribbon worn by members of the order of the Garter;
          -- hence, a member of that order.
      (b) Anything the attainment of which is an object of great
          ambition; a distinction; a prize. "These
          [scholarships] were the --blue ribbon of the college."
          --Farrar.
      (c) The distinctive badge of certain temperance or total
          abstinence organizations, as of the --Blue ribbon
          Army.

   Blue ruin, utter ruin; also, gin. [Eng. Slang] --Carlyle.

   Blue spar (Min.), azure spar; lazulite. See Lazulite.

   Blue thrush (Zool.), a European and Asiatic thrush
      (Petrocossyphus cyaneas).

   Blue verditer. See Verditer.

   Blue vitriol (Chem.), sulphate of copper, a violet blue
      crystallized salt, used in electric batteries, calico
      printing, etc.

   Blue water, the open ocean.

   Big Blue, the International Business Machines corporation.
      [Wall Street slang.] PJC

   To look blue, to look disheartened or dejected.

   True blue, genuine and thorough; not modified, nor mixed;
      not spurious; specifically, of uncompromising
      Presbyterianism, blue being the color adopted by the
      Covenanters.
      [1913 Webster]

            For his religion . . .
            'T was Presbyterian, true blue.       --Hudibras.
      [1913 Webster]