Blueprints are a new addition to BuildCraft 3.1. They offer the ability to make a complete exact copy of any structure down to the material. They differ from templates in the sense that what you blueprint will be material specific, not whatever you give the builder. If you had a wood house, with cobble stairs, glass pane windows and fencing, it would make all of that as long as you give the builder the right materials. When crafted with the recipe below, you will get what is called a 'Blank Blueprint.' You will use this with what was called the Template Drawing Table which is now called Architect Table (as of 3.1.0).
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[1]A blueprint is used in an architect table to map a structure to it, and then placed in a builder to construct that strcuture.