Hey,
I’m assuming you are not mirroring your output displays. When there is no video playing from QLab, then control of the output is released to the Mac op system so it will display whatever would be on that screen. You can change that to a completely black screen by changing the background to black. You can do this for a particular screen in System Preferences under "Desktop & Screen Saver.” If you want to black out all the desktops, QLab has a handy little option under the Tools menu to "Black out desktop background."
The other thing you need to do is go into the System Preferences under MIssion Control and check the "Displays have separate Spaces" box. That will get rid of the menu bar at the top. You will need to at least log out and back in for this to take effect. A reboot would work as well.
Hope that helps!
Mike Post