How can I turn off the screen on the macbook when I have an external monitor set up? I have an external monitor and want my macbook screen to go blank or black so I can focus on the monitor instead. On my windows machine Function F5 does this.
Turn on your MacBook without the external display connected.

Close the lid and it will go to sleep.

Connect your external monitor and an external keyboard via USB.

Now wake it up by typing on the external keyboard. If you don't have a USB keyboard (you might be using a BT keyboard and mouse) you can use the Apple Remote to wake up a MacBook when its lid is closed.

The MacBook should wake up with the external monitor as its one and only monitor.