At least for me, the GUI brightness controls only allow the brightness to be dimmed so far.

$ cat /sys/class/backlight/intel_backlight/brightness 
618

By editing that file, you can bring it down further.

echo "1" | sudo tee /sys/class/backlight/intel_backlight/brightness

For me, the value 0 works fine as well, but it doesn’t appear to be any different than 1, and I get this weird feeling that some XFCE is going decide to black out the screen on 0 someday so I just play it safe with 1.