Entering a room that requires a Key does not consume a key, but brings Isaac to a room resembling ???, which acts as an in-between of the two rooms.
These rooms contain assortments of Blue Gapers, Blue Boils, and/or Blue Conjoined Fatties, and are always long rooms (either normal or slim). They have a chance to give a reward when cleared, like normal rooms.
Once exited, the room cannot be re-entered.
Notes
Doors that were unlocked with the trinket have a blue hue emitting from them instead of the normal floor-dependent hue.
The doors leading to the entrance of Downpour or Dross can be unlocked for free by Blue Key, though the blue room generated will be a dead end. Leaving the way Isaac came in will leave the door unlocked, allowing the player to continue normally.
Rooms that require money to enter (Arcades or rooms affected by Pay To Play) can also be opened for free.
Doors that require two keys to open, such as Vaults or Dice Rooms, will be opened instantly instead of waiting for the animation of both keys to be used.
The door leading to Hush will also produce a blue room, despite not requiring a key.
Isaac can unlock the rooms with Blue Key, but not enter them straight away. By exiting and re-entering the room, he can pass through the door that was unlocked with the Blue Key without having to go through the room in between.
Synergies
Door Stop / Mercurius: The blue room can be exited without fighting the enemies, and the door will remain unlocked.
Interactions
Broken Padlock: Doors opened via explosions will not spawn the corridor.
Ventricle Razor: Any portals inside the hall will be deleted upon exiting. While in the hallway, any portals to the outside can still be taken.
The trinket might be a reference to SCP-860, a fictional key that acts in a near-identical manner.
If used on the ??? floor, one might hear the mid-combat layer of the floor's theme, Nativitate, an asset which was previously unused.
The combat layer of Nativitate can also be heard in a Crawl Space or an I AM ERROR room with enemies, or in rooms created by Red Key (or similar effects) on the ??? floor.
Bugs
Bug!
If used with The Forgotten and Birthright, leaving The Forgotten's body in the hallway will cause it to reappear next to you when you exit, but upon switching back to the body, it will become immobile, and switching back to the soul will make it immobile too, causing a softlock requiring you to exit then continue the run.
Bug!
Opening a locked door without entering it and then leaving and re-entering the room will no longer cause the door opened by Blue Key to lead to a Blue Womb room. In other words, this removes the downside of the trinket.
Key does not consume a key, but brings Isaac to a room resembling 
???, which acts as an in-between of the two rooms.
Blue Gapers, 
Blue Boils, and/or 
Blue Conjoined Fatties, and are always long rooms (either normal or slim). They have a chance to give a reward when cleared, like normal rooms.
Downpour or 
Dross can be unlocked for free by Blue Key, though the blue room generated will be a dead end. Leaving the way Isaac came in will leave the door unlocked, allowing the player to continue normally.
Arcades or rooms affected by 
Pay To Play) can also be opened for free.
Vaults or 
Dice Rooms, will be opened instantly instead of waiting for the animation of both keys to be used.
Hush will also produce a blue room, despite not requiring a key.
Door Stop / 
Mercurius: The blue room can be exited without fighting the enemies, and the door will remain unlocked.
Broken Padlock: Doors opened via explosions will not spawn the corridor.
Ventricle Razor: Any portals inside the hall will be deleted upon exiting. While in the hallway, any portals to the outside can still be taken.
Crawl Space or an 
I AM ERROR room with enemies, or in rooms created by 
Red Key (or similar effects) on the ??? floor.
Bug!
Keys 
Tainted Cain Unlocks 
The Beast Unlocks 
The Binding of Isaac: Rebirth
