I read this 20 or more years ago. It had something to do with young people (maybe boys, girls or both) being lured into a world where everything was darkness, like a negative world. The one thing I remember clearly about it is that light would destroy things, and one child (trying to get out was in a classroom watching a filmstrip) tried to knock over the projector to start a fire, but it only created black flames.
I don't even remember if it was a book or a story, but I do remember it was aimed at young adults. I want to say it was a paperback book.