The earliest place you can earn this achievement is the beginning of the second mission. There is a secondary mission to find a black market shop. Follow the marker toward the left from the docks. From the black market, if you exit to the left and through the window you can find the back door of the store, but it's locked. Instead turn right out of the black market, through the locked door (you can unlock this from the inside), then left and up the stairs.
In front of you is a large condemned building; there is a body in one of the lower windows. You can jump through this window to find yourself in a building infested with blood flies. If you move slowly and stay away from the nests, they shouldn't attack you. If they do start attacking you, they will leave you alone if you back off slowly (or you can just swing your blade and try to kill them) Slowly move your way in the right side of the room and over the counter. Turn left and head toward the stairs and take them as high as you can go (two floors). Leave the stairwell, turn left and head through the hole in the wall. You should see a piano over to your left. Head toward it, then turn left and see a hole up above a bookshelf. Climb through that, forward, and left into a room.
There is a "zombie" type guy in this room, so either kill or subdue him. Straight ahead, on the right side is a large aquarium. Inside of this is a key to the black market store. You have to break the aquarium to get it, likely causing some of the flies to attack you. Get the key and back away, then slowly work your way back out the way you came in.
Head back to the black market shop and its back door. The achievement unlocked for me the moment I entered the shop (even before I took anything).