Long story short: in Fedora 20 (and as far as I remember also 19 and 18) XScreenSaver doesn’t power off the monitor backlight when locking the screen.
Being the lazy ass I am it took like me 2 or 3 years to find the motivation to solve the issue.
It was actually pretty simple, no need to edit some obscure config file or else, in XFCE just click on: Application menu -> Settings -> Screensaver.
A window will appear, select Blank Screen Only in the Mode dropdown menu, then switch to the advanced tab, un-check power Management Enabled and check Quick Power-off in Blank Only Mode.
This is it, now every time the screen locks the monitor backlight will also power off.