The Cyanotype process was discovered by Sir John Hershel in 1842 and used to duplicate drawings and notes as “blueprints”. This was used well into the 20th century for the same purpose. A cyanotype’s primary characteristic is its deep blue color. 

A cyanotype is very archival. Images produced in the mid 1800’s have retained their original tones and densities. For such a humble process, cyanotypes really are “true blue”.