Of course, the "blank screen" really should be an "off" screen, not just a black image. Showing a black image wouldn't save any energy if the backlight is still being powered for normal brightness. (it would on a CRT--see blackle--but not with LCD.) you might as well display the desktop if the screen is going to wake up to full brightness before showing the clock. and in that case,
you might as well just show the built-in system clock on your desktop or install statusbar clock (as others have suggested) and save the processor the cycle or 2 that it takes to show a clock on a black background like emjayes' app does.

The most power-efficient solution, of course, would be to flash the clock on the screen for only 1-2 seconds and then immediately shut the screen off afterwards. AND/OR show the clock at a diminished brightness level AND/OR show the desktop at diminished/zero brightness. (perhaps this is what you were suggesting already....)

i'm curious. @emjayes: are you making this for power-saving purposes too? or just for functional/accessibility purposes?