A blueprint was a working drawing in architecture and engineering. Blueprinting was a negative process that produces prints with a white image on a blue background from a drawing or subject having dark lines on a light background. The general method of making such prints was discovered by Sir John Frederick William Herschel in the 1840's.
In the blueprint process, paper is made sensitive by being treated with a mixture of ammonium ferric citrate and potassium ferricyanide.
The paper then is exposed to light under transparent paper on which the material to be transferred is drawn. Light passes through the drawing and causes the ferric salt not covered by lines to react with the ferricyanide and form a blue background. After exposure, the blueprint is washed with water and treated with a potash (potassium dichromate ) solution to intensify the blue background. It is washed again with water to remove the potash, and then dried and trimmed. Later blueprint machines performed all of these steps, with the exception of trimming, in one continuous process. Nowadays with the advances of computers and printers it's superseded within the industry.