Use the VoiceOver screen curtain on Mac
Prevent people from seeing what’s on your screen by using the screen curtain, which turns the screen black.
Note: VO represents the VoiceOver modifier.
Turn on the screen curtain: Press VO-Shift-F11, or triple-tap with three fingers on the trackpad.
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.