Black Market Code is an audiograph found in Dishonored 2.
Transcript
This whole setup is a huge pain in the ass. Timing windows that ignore the tides, fake boat manifests, and private door codes. It might be easier just to play it straight, pick up and deliver the goods. Anyway, so I don't forget it, the code for the black market shop is Serkonan Wine is Better Than Tyvian Prisons. Idiots.
Location
The audiograph can be found during the mission The Grand Palace. Originally, it is located in a boat near the beginning of the mission. If the boat has been raided, it will instead be on a desk in a small apartment with training mannequins on the balcony, near the wall of light blocking the entrance to the Grand Palace.