For Xorg, I think
xset s off -dpms
should work.
For CLI only, try
setterm -blank 0
To completely turn off the display (needs rpi-vc.tcz to be loaded)
tvservice -o
... and to turn it back on
tvservice -p
NOTE: after turning the display off, you will have to blindly enter the command to turn it on again. Or via ssh or other means.
Also note, you may then have to switch VTs back and forth (CTRL+ALT+F1, CTRL+ALT+F2) to get the display activated.