Blueprints enable your workers to craft equipment.
Common blueprints: At the beginning of the game, hiring certain workers unlocks certain blueprint lines (e.g. hiring the Woodworker unlocks the Mace and Walking Stick, which begin the Maces and Staves lines). Afterwards, each common blueprint is unlocked by crafting the previous item in its line.
Do not confuse: rarity of a blueprint (common, uncommon, rare) vs quality of an equipment (normal, good, great, etc.).