There are six darkling outfits. Make sure you get them as the game progresses, because you can't return to some of them.
Potato Sack: In Chinatown as soon as you exit the subway station right in front of you leaning on a wall.
Jungle Outfit: While escaping the first helicopter encounter, you are required to go through a pool hall. Once you exit the pool hall there is a bum, to the left is a door with a vent above it. Use your Creeping Dark to go vent and into the room where you will find the Jungle Outfit and a collectible.
Sledgehammer Outfit: When going to kill the last guy on Vinnie's list (by Shrote's appartment), there is an open window with a guy monitoring him. The outfit is on the wall to the right.
Lumberjack Outfit: When you help Dana in the Canal Street Station she will have you go to an apartment in Chinatown. In the apartment room is the outfit.
Fireman Outfit: In the room where you kill the snitch for Jimmy the Grape in Grinder's Alley.
Jackhammer Outfit: When traveling to the Turkish Baths in the abandoned subway station, you will see an old subway train. Inside the train is the outfit.