Why blueprints are blue ?

Ever wondered why blueprints were blue and not black or red or any colour other than blue, well, it's because the technique in making blueprints caused the paper to turn blue.

Mental Floss delved into the history of blueprints and discovered that the blueprint process was developed in the 1800's when scientists found an easy way to reproduce documents by using ammonium iron citrate and potassium ferrocyanide as some sort of old school photocopy. 

Working:

Someone creates a drawing on translucent tracing paper or cloth. The drawing is placed over a piece of blueprinting paper, which has been coated with a mix of ammonium iron citrate and potassium ferrocyanide from an aqueous solution and dried. When the two papers are exposed to a bright light, the two chemicals react to form an insoluble blue compound called blue ferric ferrocyanide (also known as Prussian Blue), except where the blueprinting paper was covered, and the light blocked, by the lines of the original drawing. After the paper is washed and dried to keep those lines from exposing, you're left with a negative image of white (or whatever color the blueprint paper originally was) against a dark blue background.

You see, the chemicals react and makes the paper blue. And back in the day, using this blueprinting technique was obviously faster and more effective than tracing the documents. The blueprint name has stuck ever since.