This blueprint is located in the Dunwall Whiskey Distillery during the House of Pleasure mission. Once you're inside the main building, there is a room on the right with a valve, which opens the door right next to it. Turn the valve all the way, then quickly turn towards the door and use Blink to get inside, where you'll find the blueprint.
It can also be found in The Flooded District mission, in the Gateward Tunnel section. It can be found in the cellar, which is near the exit of this area. This blueprint unlocks the Incendiary Bolts for purchase at Piero's Workshop.
