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Vellum is what many know as traditional blueprint paper, and it is commonly used for architectural and engineering plans for black-and-white blueprints. It is also possible to print blueprints on Mylar paper, which is transparent or frosted, by using a laser process.

And, by the 1980s, the architecture, engineering and construction industries were making the move from hand-drawing to computer-aided design (CAD) that could be printed on large-scale paper. Today, "blueprints" aren't really blue. They are usually black or gray lines on a white background [source: Soniak].

A History of the Blueprint Once the drawing was exposed to light, the exposed parts turned blue, while the drawing lines blocked the coated paper from exposure and remained white. ?One hundred years later, in the 1940s, blueprints were replaced by diazo prints, aka whiteprints or bluelines.

Why? It's because of how those documents are made. The blueprinting process was developed in the mid-1800s, when scientists discovered that ammonium iron citrate and potassium ferrocyanide created a photosensitive solution that could be used for reproducing documents.

In this process, initially created just to transfer notes, two papers are exposed to a bright light, and the applied chemicals create a permanent blue compound, called blue ferric ferrocyanide, except for where the paper is covered or has lines drawn. As a result, a solid-blue document with white lines is created.

The two iron salts do not react with each other in the dark, but when they are exposed to ultraviolet light the iron(III) ammonium citrate becomes an iron(II) salt. The iron(II) ion reacts with the potassium ferricyanide to form an insoluble blue compound, blue iron(III) ferrocyanide, also known as Prussian blue.