Unused monitors should remain in their initial (completely off, no backlight) state.

[Péter]

#I have 2 monitors, and one of them is "out of use", i.e. the "use this display"
#(in Setting/Displays) is unchecked. (I use xfce4.)
#My used monitor is (of course) on; my unused monitor is completely off (backlight is off).
#Now I enter this:

$ light-locker-command -V
light-locker-command 1.6.0

$ light-locker-command --lock
#Okay, all my monitors turns black, all backlights turns off.

#A bit later, I push the Shift [or any] button.
#All my monitors switches on, all [in stereo] shows the password dialog. I enter the password.
#My used monitor remains on - it is okay.
#My "unused" monitor turns "dark-bluish", what is "halfly" off, but not completely.
#The backlight remains on, which is not wanted.
#The backlight of the unused monitor should not even switch on (when resuming).