I had a problem with my old monitor turn off blank after 10 minutes, no matter how I configure xscreensaver.
The solution I found, is that you must add this line at the end of the ./profile file in your /home directory:
# Turn off screen blanking xset s off && xset -dpms
Then restart. After this Your monitor will stop blank after 10 minutes.
(Do not add this line to autostart - it will not work, it work only if you put it in ./profile file in /home directory)