Beat "Blue Baby" (???) With Blue Baby.
This Means:
- Unlock Polaroid, Grab The Polaroid, Go Through The Cathedral.
- Beat The Cathedral Boss Five Times To Unlock Chest.
- Go Through The Chest To Find Blue Baby, Beat Him While Playing As Blue Baby.
Tips:
Dying And Turning Into Blue Baby With The Passive Item DOES Get You The Completion Mark With Blue Baby, Not The Character You Started With.
Dont Die. (Game's Hard)
Beating Mom's Heart 10 Times Unlocks Blue Baby For Play.
Good Luck!