Wow, it's been so long that even this room slipped from my mind.
>Find two beds that look like Sunny and Mari's, can push Mari's bed to reveal a ladder
>Under it you find a room with blurred letters
>They unblur when approached and spell I'M SORRY
Unless I'm forgetting a room which I might very well be, I remember this was my favorite Black Space room. It's so simple, but effective. Not even background ambience. Just complete silence, leaving the message as all you have.
It also starred in one of my favorite scenes from that Dreaming Boy Sleeping Girl fic, but that's not as important.