In 1704 the German chemist Diesbach mixed the shells of the Cochineal beetle with alum, ferrous sulfate and potash. He intended to make a red pigment called Florentine Lake but ended up with a blue stuff. The potash was contaminated with animal blood, and that's how the first synthetic pigment was discovered.
Prussian blue proved stable, lightfast, and cheap, so it became a popular complement to the ochres that people painted their furniture with. In watercolor it has been used since around 1730 and much appreciated for its transparency and intensity. Among its other properties it tends to make greens when mixed and dryes considerably lighter. I use it as a primary color despite the fact that it's a bit duller and greener than the real primary blue. If only one blue was allowed, Prussian would be my choice.
By the way, if you heat it up to 140 °C you get Hydrogen cyanide, perhaps better known as Zyklon B. Don't do that!