The Black Market Burglar is one of the easiest Achievements/Trophies that can be earned in Dishonored 2. And while it can can be earned at multiple points during the game, we are here to show you how to get it at the earliest possible location.
When you first start out on mission 2, Edge of the World, walk up the small flight of stairs and proceed left onto the blood soaked walkway. Follow the ramp upwards, leaving the giant dead shark below you on the docks, as you arrive at a large open area. From here, just keep going straight until you arrive at a building with a man cleaning fish next to the entrance.
Walk through this archway and make you way to the second floor to arrive at the entrance to the Black Market, which you can clearly see has a lot of useful items.
Walk out of the room and immediately turn left, heading straight for an open window. Climb out to find an NPC named Mindy Blanchard lounging on a sofa to your left, next to the back entrance of the Black Market, a door that requires a key.
Open the gate to your right and keep moving until you encounter a dead body hanging out of a window of a Bloodfly infested building. Make you way up the floors of the building until you reach a hole in the wall that leads to a piano.
To the left of the piano is a blocked door, which you can still proceed through by climbing up on a cabinet. Do so, and follow the path to the only room available. Dispatch of the Nest Keeper enemy and you will come face to face with a glass case filled with Bloodflies, and the Black Market Shop Key. Grab it.

Head back to the door shown above, knock out the shopkeeper and plunder to your heart’s content. You will now receive the “Black Market Burglar” Achievement/Trophy.
Released on 11th November, 2016, Dishonored 2 is available now on the Xbox One, PlayStation 4 and the PC.