why cobalt is blue?

Cobalt blue is a cool, slightly desaturated blue color, historically made using cobalt salts. Because of its chemical stability in the presence of alkali, cobalt blue is used as a pigment in blue concrete. The blue seen on many glassware pieces is cobalt blue, and it is used widely by artists in many other fields. "Cobalt Blue" is used as a filter used in ophthalmoscopes, and is used to illuminate the cornea of the eye following application of fluorescein dye which is used to detect corneal ulcers and scratches.