"A cyanotype is a photographic printing technique and one of the oldest processes in the history of photography."
Cyanotype is one of the oldest photographic processes, created with a simple solution of ferric ammonium citrate and potassium ferricyanide. The oxidation of potassium ferricyanide produces the pigment known as “Prussian Blue”, which accounts for the signature blue of cyanotypes.  The characteristic blue color of cyanotypes is where the name "blueprint" originates.

Educational information sourced from https://scma.smith.edu/