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 information as of the latest Repentance release.
To see latest up-to date information about this trinket for the new Repentance+ (Online Co-op) DLC, 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.

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