Blue Key is an unlockable trinket in The Binding of Isaac, added in the Repentance DLC.

Entering a room that requires a key or coin does not consume the key or coin, but instead takes Isaac to a newly generated Blue Womb room that is not a part of the current floor's layout, but acts as an in-between passage of the 2 rooms.

These rooms contain assortments of Blue Womb-styled enemies, such as Blue Gapers, Blue Boils and Blue Conjoined Fatties, and are always long rooms.

The rooms can give a reward when cleared, just like normal rooms.

Once exited, the blue room cannot be re-entered.

The door leading to The Hush fight will also create a blue passage room.

This page shows the latest information as of the new Repentance+ (Online Co-op) DLC.
To see information about this trinket for the previous Repentance version, check this page instead.

Defeat The Beast as Tainted Cain.

The following special synergies and interactions for Blue Key have been documented.

* Note that not all possible synergies and interactions are listed here.

Below is the Internal Item Description as shown in-game.
Note that some stat changes are not displayed here. Please see the main description for the full effects.

Blue Key

• Locked doors can be opened without a key
• A ??? room appears before the destination
• Golden Trinket (without Mom's Box): Rewards for clearing these rooms are doubled
• Mom's Box: Rewards for clearing these rooms are doubled
• Mom's Box + Golden Trinket: Rewards for clearing these rooms are tripled

• Check out the trinket description on the external Isaac Wiki.