You could drive yourself crazy @Bobby115 trying to figure out why other people do the things they do. They're paranoid about someone looking in the windows? They like it dark? They're trying to get the most out of the money they spent by using your electric lights all day, even when it's bright and sunny out? They're actually vampires?
You could take a humorous approach and ask them. "I'm doing a little survey among my guests, as I notice many of them keep the blinds closed all day and maybe I have a paucity of imagination, but it bamboozles me. If you wouldn't mind participating, could you tell me your reason for keeping the blinds closed all day? No problem that you do, I'm just trying to broaden my understanding."