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.
Door Stop/Mercurius: The blue room can be exited without fighting the enemies, and the door will remain unlocked.
The trinket is most likely a reference to SCP-860, a fictional key that acts in a near-identical manner.
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.