Turn a single screen blank (main display)

[RomZ99]

I want to turn off/sleep the main display while the extended display is playing a video in full-screen and I want to keep it as a setting as I have a top-bottom monitor setup.

[waydabber]

If you both want to keep it in the layout and still turn it off, your only option is to either darken the display fully (if it is an OLED or mini-LED display) or use DDC standby - if the display supports DDC power controls (not all displays do, some faulty ones can't turn the panel back on).