In Debian 10
gnome-screensaver-command -a can be used to enable blank screen (I set this up as a shortcut to Win-Shift-L). Mouse movement and modifier keys will turn the monitors back on but won't exit gnome-screensaver unless a normal key is pressed or a mouse button is clicked (or dragged?). Monitors turn back off after a very short delay while in gnome-screensaver.
Blank screen can also be done automatically after a delay the same way you would normally lock the screen automatically, just disable Automatic Screen Lock in Gnome Settings
Unfortunately the curtain appears to go away on mouse movements alone (and even other reasons?) in Gnome 3.38 (Debian 11 Bullseye); the net effect is that if anything unblanks the screen I get back to the desktop with normal blank timeout (set at 10 minutes for me, and this happens quite often even while I'm not at my computer at all) instead of blanking back after just a few seconds. I'm still looking for a way to re-enable it.