Unlocks the passageway to ???, regardless of run duration.
If Pandora's Box is used while holding Strange Key, the following effects will occur instead of Pandora's Box regular effects:
The normal effect of Pandora's Box for that level is ignored, and instead 6 generally higher quality items from a set of item pools are spawned.
Both Pandora's Box and the trinket are consumed.
The items are chosen from the following item pools: Treasure Room, Shop, Boss, Devil, Angel, Secret Room, Library, Golden Chest, Curse Room and Planetarium.
The Treasure Room item pool has a much higher chance to be chosen while the others have an equal chance.
Both effects work even if the key has been turned into a passive item from items such as Smelter or Marbles.
Using Pandora's Box with the key in this state will remove the key from Isaac's Inventory.
If playing with modal characters such as Tainted Lazarus, the character with the Strange Key must enter the room while holding it to cause the passage to open. Leaving and re-entering the room after switching to the correct character will open the door normally.
Strange Key is likely a reference to the film by David Lynch, Mulholland Drive (2001), where a small blue box being unlocked by a strange key is a major plot point.
Strange Key's role in opening the Blue Womb is similar to that of The Polaroid when it was first added in Wrath of the Lamb 1.3. Like Strange Key, the Polaroid was originally a trinket that could only be found randomly, although since there is no alternative way to reach the Chest, this was changed in Wrath of the Lamb 1.4 so that Mom would drop it.