VoiceOver Utility (Mountain Lion): Turn the screen black using screen curtain
For added privacy, you can prevent those around you from reading the screen by using the screen curtain, which temporarily turns the screen black. The screen curtain is also useful for sighted users who want to avoid reading the screen while learning to use VoiceOver.
To turn off the screen curtain, repeat the command or gesture.
If you turn off VoiceOver while the screen curtain is on, the screen curtain is also turned off.