If you are using Gnome:
AFAIK the blank screensaver is not intended to turn the screen off, just to provide a fallback if no other screensavers are installed or if the warm-up time on your monitor is long but burn-in is a problem.
If you want to save power, as alan99 said then use the 'Put display to sleep when inactive for' feature in Gnome Power Management ( System --> Preferences --> Power Management --> On Battery Power Tab ).
Last edited by William (Dthdealer); 07-23-2010 at 06:11 AM.
Reason: Added gnome line