How did blue prints get its name?

Back in the days people needed something to produce several copies of a document quickly. This can be done using a contact print process on light-sensitive sheets, often negatives.

In 1861 Alphonse Louis Poitevin found that ferro-gallate in gum is light sensitive. A coating of this chemical on a paper or linen was used to reproduce an image from a translucent document. Light however turned the document permanent blue and the process was unable to reproduce color or shades of grey.

The "Blue Prints" was therefore at that time blue sheets with light lines.