The Collaborative International Dictionary of English v.0.48:

Cobalt \Co"balt\ (k[=o]"b[o^]lt; 277, 74), n. [G. kobalt, prob.
   fr. kobold, kobel, goblin, MHG. kobolt; perh. akin to G.
   koben pigsty, hut, AS. cofa room, cofgodas household gods,
   Icel. kofi hut. If so, the ending -old stands for older
   -walt, -wald, being the same as -ald in E. herald and the
   word would mean ruler or governor in a house, house spirit,
   the metal being so called by miners, because it was poisonous
   and troublesome. Cf. Kobold, Cove, Goblin.]
   1. (Chem.) A tough, lustrous, reddish white metal of the iron
      group, not easily fusible, and somewhat magnetic. Atomic
      weight 59.1. Symbol Co.
      [1913 Webster]

   Note: It occurs in nature in combination with arsenic,
         sulphur, and oxygen, and is obtained from its ores,
         smaltite, cobaltite, asbolite, etc. Its oxide colors
         glass or any flux, as borax, a fine blue, and is used
         in the manufacture of smalt. It is frequently
         associated with nickel, and both are characteristic
         ingredients of meteoric iron.
         [1913 Webster]

   2. A commercial name of a crude arsenic used as fly poison.
      [1913 Webster]

   Cobalt bloom. Same as Erythrite.

   Cobalt blue, a dark blue pigment consisting of some salt of
      cobalt, as the phosphate, ignited with alumina; -- called
      also cobalt ultramarine, and Thenard's blue.

   Cobalt crust, earthy arseniate of cobalt.

   Cobalt glance. (Min.) See Cobaltite.

   Cobalt green, a pigment consisting essentially of the
      oxides of cobalt and zinc; -- called also Rinman's
      green.

   Cobalt yellow (Chem.), a yellow crystalline powder,
      regarded as a double nitrite of cobalt and potassium.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cobalt blue
    n 1: a shade of blue tinged with green [syn: greenish blue,
         aqua, aquamarine, turquoise, cobalt blue, peacock
         blue]
    2: greenish-blue pigment consisting essentially of cobalt oxide
       and alumina [syn: cobalt blue, cobalt ultramarine]