Blueprints - how to find blueprints | Collectibles Dishonored Guide

The blueprints can be bought from Griff or, you can find them during your gameplay. Finding a blueprint is not enough, as you return to Piero in the pub, to use it, and give the blueprints to him. This will result in the enrichment of the stock offered by him by a new piece of equipment or an upgrade to Corvo's gear.

Note - If you missed any of the blueprints earlier in the game, you do not need to go back to that location to get it. The game gives other opportunities to obtain this particular blueprint during the next missions.