I have a iMac that is connected to an external monitor to extend my screen. Into that same monitor I have a separate Windows box that I need to run sometimes. The Windows box goes in through VGA and the iMac through HDMI. The problem I'm running into is that when I want to use the monitor for Windows, there is no simple way to get my iMac back to just that monitor. If I leave it extended, some apps are open in the other 'monitor' and I can't access them without work arounds. If I turn on mirroring, the resolution on the iMac drops significantly, even if I tell it to keep the iMac screen size. I can't turn the other monitor off as that defeats the point. I have to unplug the HDMI to get it to perform correctly. Is there any way (command line, an app, preferences, etc) to toggle between using the display and not on my Mac?
There is no way to "virtually" disconnect a display on a Mac, like you can on Windows. The Mac needs to disconnect physically. The easiest way to do this is by purchasing an inexpensive KVM Switch, to physically switch between the Mac & PC.
Use VLC player to play your movies. Go to Preferences → Video and choose black screen when in full screen.