If all you want to do is blank the screen, as opposed to running a fancy animation or locking the screen, then
xset can do it.
xset dpms force off
If you want to lock the display, you need a screen locking program, e.g.
xscreensaver-command -lock -activate or
gnome-screensaver -al or (for KDE4)
qdbus org.freedesktop.ScreenSaver /ScreenSaver Lock.
If you want to turn off one monitor only, you can to it through
xrandr (unless you're using a proprietary display driver, in which case you'll have the tools provided by NVidia/ATI).
xrandr --output DVI1 --off
This won't be undone by moving the mouse, you need to run
xrandr --output DVI1 --auto to bring the monitor back.