Your first idea was better, but would take a lot of custom sprite sheets (or a lot of really good luck). My idea would be make an event, make a black sprite sheet (semitransparent), and set it up through events, like the above said, but have a spritesheet for every room in the game. Yeah, it'll take a bit of memory, but it's the most safe way to do it. You just place that event that holds the sprite sheet right in the center of the room (make sure it's the same size, or a little bit bigger, as/than the room or it'll look... strange) you want to black out.