We have the reverse problem to the OP in this question (Mac Dual Monitor: Prevent Second monitor to go black while first one is fullscreen). He is asking how to prevent black out of the primary display when there is a fullscreen app on the secondary display. Wish we had this problem...
We want the primary display to be blacked out when playing (VLC) full-screen on the secondary screen. (The primary screen is the iMac itself, the secondary screen is a MiniDisplayPort to HDMI connected projector.)
What we tried:1. F2 to lower brightness – works somewhat, but only to low brightness not to blackout.2 "Display have separate Spaces" enabled or disabled – does not work; in both cases, the primary screen does not go to black.3. The shortcut Shift-Control-Esc – does not work, sets the secondary display (or both) to black (not the primary display even the active window and the mouse is on the primary display) 4. "Hot Corners: Put display to Sleep"– does not work, blacks out both screens.
How can we black out only the main (internal, primary) screen?
At the moment, I am putting VLC in separate space and have set a black desktop background, but there must be a better solution...
Cheers!