especially on mobile or all-in-one desktops like the iMac you might want to turn off the Display manually. For example if a long dd command is running.
To do so, you can use the small command xset with which you can control many display and energy saving tasks.
xset dpms force off
xset dpms force on
or you could just move the mouse or press any key on the keyboard
You can also use xset to set the timeouts for Screenblacking Screensaver. You should take a look in the _man_pages of it