Why are blueprints blue?

Blueprints are not actually blue. The name “blueprint” is a misnomer. The original prints were made using a light-sensitive paper called ferroprussiate paper, which was coated in a mixture of iron salts. When exposed to light, the iron salts would turn blue, creating a positive copy of the original drawing. The term "blueprint" stuck even after the introduction of other reproduction methods, such as diazo printing, which produces black-and-white prints.