Blue print is also known as amino print.
As printing to those drawing with help of ammonium so no termite or rats cuts the drawing. The drawing are make to the scale and scale bar is also marked on it. As the paper sharing due to time the actual drawing shrinks and we can not measure the distance directly. So bottom part of scale markup can be used for the same.
And in old days they have to read the drawing in night also. In night the blue print is more clearly visible in less light than black and white print. Some old British has used the light in which blue print shins like radium.