Ok, so forget most of what you've read on the internet regarding this - lots of the "fixes" out there refer to display blanking while in text/console mode or for older versions of the X server, or additions to your xorg.conf file which don't work anymore. Plus, the fixes generally don't survive a reboot or a restart of the X server, others won't take effect until you do restart the X server. Argh!
But, remember, I said to forget about those? Yeah. So here's what finally seems to have solved it for me. You use your administrative privileges (you have those right?) and open the following file in your favorite text editor:
At the end of it tack on the following two lines:
xset s off
Save the file, reboot. That *should* (if you have every other screen blanking/saving feature turned off) prevent it from blanking out the screen after several minutes while watching online videos.
Some Linux players, such as XBMC, don't require this fix as they implement other methods to prevent the screen blanking when you haven't touched the mouse or typed anything in a while.