There is a bug in Win8 that prevents it from putting monitors into sleep mode correctly. It's hit and miss though, sometimes it works sometimes it doesn't. Don't waste your time formatting or reinstalling, it doesn't fix it.
Basically what happens is, the monitor DOES turn off, for about 30 seconds-1 minute, then sometimes it will turn back on with a black screen. Other times it stays off. There isn't any pattern to it.
Good luck waiting on Microsoft to fix it though, I doubt they will even acknowledge it, they will just say "check your power settings". Maybe it'll be fixed in Win9.