Macs don't make it very easy to choose a black background on your computer, so when you're projecting you need to point the desktop to a black image file. Put this in your pictures folder on your computer, then point to it by going to System Preferences>Desktop & Screen Saver. You'll need to repeat this for each external projector you plug in.
This also helps when you're trying to limit the amount of ambient light in the control booth.
