The chemical composition of this natural pigment: hydrated iron phosphate with the formula of Fe3(PO4)2 · 8H2O. On exposure to air, the Fe2+-ions can be oxidized to Fe3+ which causes a color shift towards darker blue and blue-green.
Blue ochreNamed after either John Henry Vivian, a Welsh-Cornish politician, mine owner and mineralogist living in Truro, Cornwall, England, or after Jeffrey G. Vivian, an English mineralogist.From Wikipedia
The pigment is prepared from naturally occurring mineral by breaking, grounding and purifying. The whole process is shown in the video from Master Pigments below.
It was used as a pigment since antiquity but its use in oil painting remained rare.