Welcome to the Support Communities! The article below will help you troubleshoot the issue you are having with the 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.
Press VO-Shift-F11. If you’re using VoiceOver gestures, triple-tap with three fingers.
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.
I hope this information helps ....