I have completely exhausted all possibilities. The only remote possibilities are the room 1 left and below the starting room, which has a door which expects a password which I don't have, and the special blocks with checkmark characters on them which turn red briefly when you detonate a bomb next to them. But dozens of bombs don't blow them up. I have not found a blank key to turn into an orange key with 50 coins in the shop. I eventually bought the complete map, which was completely useless, and also left me with less than 50 coins so even if I did find the blank key I couldn't get it since I've killed all enemies in all accessible rooms so there's no more coins to be had without getting access to more rooms. But that's not possible anyway, since there is none in any of the accessible rooms. I'm SOL and I'm tired of this.