A deep, rich shade of
blue, much darker and more substantial than
sky blue. My dictionary describes it as greenish-blue, but I'd always thought that "cobalt blue" referred to a very dark, very
pure blue, like the color of
Skyy Vodka bottles (or any other dark blue
glass, really).
According to one source, Cobalt Blue under the Munsell System is: Hue 6PB, Value 3.00, Chroma 11.0, which sounds pretty close to what I've described (although I've got no handy reference to see what that shade actually looks like).
The name comes from a fact that glass attains this kind of hue when doped with cobalt and some optional aluminum oxides. (SharQ further notes that most of the world's cobalt once came from Blåfarveverket in Norway.)
There are such things as cobalt green and cobalt yellow, too, but I doubt they're as lovely as the blue. Which would explain why "cobalt blue" is so much more common a phrase.
Source:
-- Golden Artist Colors Inc.: http://www.goldenpaints.com/munsell.htm