Blue Key

Effects

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.

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) Downpour or (Dross) 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 ((Arcade) Arcades or rooms affected by (Pay to Play) Pay to Play) can also be opened for free.
Doors that require two keys to open, such as (Vault) Vaults or (Dice Room) 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) Mercurius: The blue room can be exited without fighting the enemies, and the door will remain unlocked.