why are blueprints blue?

Answer 1

Originally "blueprints" were white-line drawing copies printed on a blue background by contact exposure of a pencil or inked original drawn on paper or vellum.

"The blueprint process was developed by the British astronomer Sir John Herschel in 1840. The photosensitive compound, a solution of ammonium ferric citrate and potassium ferricyanide, is coated onto paper. Areas of the compound exposed to strong light are converted to insoluble blue ferric ferrocyanide, or Prussian blue. The soluble chemicals are washed off with water leaving a light-stable print."

Note this is just the opposite of the Diazo process, which leaves blue lines on a white background.

Answer 2

Originally the "copy process" which was similar to mimeography produced white on blue copies. If you are old enough to remember test day at school and the euphoric smell coming from the office, you have a sense of the blue in blue prints.

Answer 3

It was a older printing way using a single color to produce a engineering designs on paper.

Answer 4

It used to be a limitation of the copying process. Nowadays you can make them any color you like. Our office makes them black print on yellow background.

Answer 5

Diazolid salts turn blue when exposed to amonia. They turn white (burn away when exposed to light) Now we use xerox machines in black and white