NOTE: This issue has since been fixed, as far as I am aware (it works now). If you are seeing something else happen, this may not be it. That does, of course, assume that you haven't edited the config in a way that disables this behavior.
In the past, the Raspberry Pi computers could be made to turn off the monitor (as opposed to leaving the backlight on with a blank/black screen) when the Pi turned the screen off on inactivity. This could be done by placing the (poorly documented?) line
hdmi_blanking=1 in /boot/config.txt, but this no longer works. I now cannot even find any reference to this command via Google search OR by looking at the RPI references on their site, and it has no effect on my Raspberry Pi 4 -- the screen goes blank, but remains on.
I have had no success even finding someone else asking this question, or determining what method to use to fix it. It does not help that there are many similar answers for different OS and Pi versions that no longer apply...
Has this been replaced by another command, or is it not implemented yet (in the 4's drivers, or the new open-source stack)? I have not checked whether it still works on older Pis, so I can't say whether it's 4-only.