I second that. Mirror screens.
Dim main screen to darkness and that is the best you can do without a potential software hack. There is no setting that is user accessible for this. There MAY be a hack for this, but I am unaware of one offhand. IME, hacks of this type do lead to potential instability.
I have my iMac hooked up to anice Cinema display and th iMac is tucked away out of sight with the same dimmed configuration.
I needed the features of the iMac without the cost of the MacPro with the screen real estate of the Cinema. The mini was too underpowered. So I am that mythical mid tower Mac users without a product.