Video - Why are blueprints blue?
No, it's not because architects really like the color
Blueprints are blue, right? But why?
(No, it's not because architects really like the color.)
(And no, it's not because architects are really sad people.)
It's because in 1842 Sir John Herschel invented a method to easily copy drawings, using potassium ferrocyanide and ammonium iron citrate. The exact method is called cyanotype and ...
... well, let's Craig Benzine (aka WheezyWaiter) explain it better in this interesting video.