On ACDs the brightness does not lower to an essentially dark screen like a PB, its still pretty damn bright when on the lowest setting. Kind of comparable to a PB screen on 80% or so.
I dunno how you could do that other than have the energy saver "display sleep" setting at a very short amount of time.
I don't know of any setting you can change, but maybe there is a software option out there. Some little app or widget that allows you to click a button to sleep the monitor.
Additionally, you could set the screen saver to nothing and utilize a hot corner so that you can automatically activate it whenever you want. That way it could be dark or swirly or whatever you want.