Make all screens but 1 to go dark

I have a MacPro ("ashcan") that supports up to 6 monitors. I have multiple large-screen monitors running right now, but I'd like to be able to have all but one go dark (black), such as when I'm watching a full-screen video on one monitor. The others being on are quite a distraction so I'd like to "blank" them for that purpose.

Is there any keyboard shortcut (or other way) to do this (on Mojave)? 

- I know the ctrl-shift-eject combo but that causes all screens to go dark. 
- And if I *gasp* turn off the other monitors macOS thinks they're no longer connected and scrambles all of my carefully placed windows onto one.
- I'm skipping the upgrade to macOS Catalina completely. It's Horrorville.

-Carl