ferric ferrocyanide

Prussian blue is a dark blue pigment with the idealized chemical formula Fe 7(CN) 18. To better understand the binding situation in this complex compound the formula can also be written as Fe 4[Fe(CN) 6] 3 · xH 2O. Another name for the color is Berlin blue or, in painting, Parisian or Paris blue. Turnbull's blue is the same substance, but is made from different reagents, and its slightly different color stems from different impurities.
Prussian blue was the first modern synthetic pigment. It is employed as a very fine colloidal dispersion, as the compound itself is not soluble in water. It is famously complex, owing to the presence of variable amounts of other ions and the sensitive dependence of its appearance on the size of the colloidal particles formed when it is made. The pigment is used in paints, and it is the traditional "blue" in blueprints.
In medicine, Prussian blue is used as an antidote for certain kinds of heavy metal poisoning, e.g., by thallium and radioactive isotopes of caesium. In particular it was used to absorb 137 Cs+ from those poisoned in the Goiânia accident. Prussian blue is orally administered. The therapy exploits Prussian blue's ion exchange properties and high affinity for certain "soft" metal cations.
It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. Prussian blue lent its name to prussic acid (hydrogen cyanide), which was derived from it. In Germany, hydrogen cyanide is called Blausäure ("blue acid"), and Joseph Louis Gay-Lussac gave cyanide its name, from the Greek word κυανός (kyanos, "blue"), because of the color of Prussian blue.