Why are blueprints blue?

Answer 1

These days, blueprints are apt to be digital. Time was, technical drawings were done in pencil or ink on large sheets of paper and copied on ammonia process machines that produced a blue-on-white print.

Answer 2

Actually, they aren't always blue. In the old days blue prints could be drawn on carbon paper that allowed multiple coppies of a plan to be made at once. Today most "blueprints" are printed from a computer on white paper and multiple copies can easily be plotted.