cyan n.

word-forming element used in science for the carbon-nitrogen compound radical, from a Latinized form of Greek kyanos "dark blue" (see cyan).

The immediate source of its use in science is French cyanogène, the name given to the compound radical by Gay-Lussac. He called it that because it first had been obtained by heating the dye pigment powder known as Prussian blue (see Prussian).

The cyanogen radical was one of the first examples of a 'compound radical' and was of importance in the development of structural chemistry during the next forty years. [Flood, "Origins of Chemical Names"]  

"blue disease," the "blue jaundice" of the ancients, 1820, Medical Latin, from Greek kyanosis, from kyanos "dark blue color" (see cyan) + -osis. Also cyanopathy. It is caused by imperfect circulation and oxygenation of the blood.