ok google dictionary has it wrong. 

Wikipedia has a better definition.

You see back in the day we used to have to make drawings by hand.  We would start with a pencil and then maybe go over with a pen.  There where no computers and printers.  So when you needed 95 copies of a drawing to send out to the customer and various contractors to be able to bid on a project instead of drawing 95 copies some chemist came up with this blue die crap that would make a blue copy of a drawing leaving all the lines white.  So instead of running one print around to 95 contractors we could now make a copy of the one print and send out a copy to everyone.  So if your are still going threw the process of making blue prints please buy a printer that can print more than 10 pages per minute, or use the save as pdf option on your drawings.  Those chemicals have been found to be kind of messy, to make them blue prints and it takes days to make them.