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.
Hold down the decrease screen brightness button at the top of the keyboard, it will get dimmer and dimmer, and at the end jump to a black screen (the screen's light will actually be off). To turn the screen back just press the increase screen brightness button.
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.