It's a cobalt oxide-aluminum oxide.
Very costly and extraordinary stable
pigment of pure blue colour discovered
by Thenard in 1802. It is now the most
important of the cobalt pigments. Although
smalt, a pigment made from cobalt blue glass
has been known at least since the Middle Ages,
the cobalt blue established in the nineteenth
century was a greatly improved one.
* Cobalt blue in impure form had long been used in
Chinese porcelain, but was independently discovered
as a pure alumina-based pigment by Louis Jacques Thenard,
in 1802. Commercial production began in France in 1807.
The first recorded use of cobalt blue as a color name in
English was in 1777. The leading world manufacturer of
cobalt blue in the 19th century was Blassfarvevaerket ("blue-color works," in Dano-Norwegian)
in Norway, led by Benjamin Wegner. Germany was also famous for
production- especially the Blaufarbenwerke
("blue-color works," in German) of Schneeburg.