The Blackmarket Weapons Locker key in Call of Duty: Modern Warfare 4’s DMZ mode can be used at two locations: Hanrin Village and Deadtown. Each site has two lockers, allowing players with multiple keys to open up to four locked containers in one deployment.
At either location, look for a building with wrought-iron gates at the front and a red MP3-player advertisement above the entrance. Go inside, find the stairs to the basement, then pass through another door to reach a room containing the lockers and other lootable containers.
The lockers can contain valuable items, including weapons, armor, crafting materials and 3D printer recipes. Both lockers can be opened if you have two keys; the room also contains other loot to collect.
Sources: Polygon.