add an alternate boss to Baby Plum that is set in the blue room, example: greasy blue.
mechanics: it moves inside its giant blue bagoneta where it joins or merges with it, attack 1: from there it will attack by opening its stomach and launching a whip of entrails. attack 2: will open his mouth to come out: a circle of Hush flies that last for a single tear / a blue yawn / blue boil (will summon a Hush fly every so often).
example: