What is Blueprint?

Legal Definition
A blueprint is a reproduction of a technical drawing, documenting an architecture or an engineering design, using a contact print process on light-sensitive sheets. Introduced in the 19th century, the process allowed rapid and accurate reproduction of documents used in construction and industry. The blue-print process was characterized by light-colored lines on a blue background, a negative of the original. The process was unable to reproduce color or shades of grey.

Various base materials have been used for blueprints. Paper was a common choice; for more durable prints linen was sometimes used, but with time, the linen prints would shrink slightly. To combat this problem, printing on imitation vellum and, later, polyester film (Mylar) was implemented.

The process has been largely displaced by the diazo whiteprint process and by large-format xerographic photocopiers, so reproduced drawings are usually called "prints" or just "drawings".

The term blueprint is also used less formally to refer to any floor plan (and even more informally, any type of plan).
-- Wikipedia
Legal Definition
The drawing of a buildings structure in the construction industry. It is made on blue paper.