Imac display off

How can i make my imac display turn off by clicking one key. I want to keep it off while changing song in itune. I am using brightness control, but is there anyway even better

No, unfortunately there's not.
Been looking into that myself recently, but to no avail.

Go to System Preferences, then Expose & Spaces. Click the Expose tab and set a hot corner for "Sleep Display".

yes but that will stop my itune, i want to turn off the screen, while listening to itunes and changing song with my controller and the screen is off

No, it will not stop the playback. It will put the display to sleep, that is all. The remote will wake the display though, but if you play through without the remote it will stay off.