The Collaborative International Dictionary of English v.0.48:

Cerulean \Ce*ru"le*an\, a. [L. caeruleus.]
   Sky-colored; blue; azure. --Cowper.
   [1913 Webster]

         Blue, blue, as if that sky let fall
   [1913 Webster]

         A flower from its cerulean wall.         --Bryant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cerulean
    adj 1: of a deep somewhat purplish blue color similar to that of
           a clear October sky; "October's bright blue weather"
           [syn: azure, cerulean, sky-blue, bright blue]
    n 1: a light shade of blue [syn: azure, cerulean,
         sapphire, lazuline, sky-blue]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "cerulean":
   Caelus, air, azure, azure-blue, azure-colored, azurean, azured,
   azureous, beryl-blue, berylline, blue, blue sky, bluish, caelum,
   canopy, canopy of heaven, ceruleous, cerulescent, cope, cyanean,
   dark-blue, deep-blue, empyrean, ether, firmament, heaven, heavens,
   hyaline, lift, lifts, light-blue, lightish-blue, livid, pavonian,
   pavonine, peacock-blue, sapphire, sapphirine, sky, sky-blue,
   sky-colored, sky-dyed, starry heaven, the blue, the blue serene,
   turquoise, vault, vault of heaven, welkin