Probably should work...What about using clear plexi, and go to a sign shop and have your 'block' made in thick white sign vinyl with a black vinyl cover over that?
Pete
OK, here's the scoop... the white is usually fairly opaque, and reflects alot of interior light around... The black (or whatever color you want) is put over it as your external finish look. Remember, what you want it to block the light...The only problem with using tranparencies is that the ink is WAY too thin and will look gray, or really be hotspotty and show where the light is. What you want is to spread the light out equally to illuminate things. That is why, with a lightpipe system, that you can etch into a piece of acrylic or PC, and light the side of the piece with a few LEDs or lamps, and the light that hits the engraving reflects the light allowing it to seem to glow. It is always hard to light the back of something and not have distinct light areas, like in the picture above. If you were trying to glow the text evenly, there are better ways of doing it. Maybe a piece of acrylic that has been sanded/blasted before the text to act as a diffuser... It can get pretty elaborate... At least LEDs don't have the heat of an incandescent bulb!!!