On some HP's you can tap the FnKey to switch from internal to external displays and cause the effect you want.
The laptop with be an HP Windows 7 machine. The screen brightness can be turned down with F keys but not off unless making adjustments in Control Panel.
Is there any software to install that will allow a hotkey for example, to be used to immediately blacken the screen? Apple's laptops have a button that will dim the screen until it's black by pressing it several times.