Is there a polymer material that is translucent from one side but not from the other side?

I want to build some custom push buttons that should appear completely black. Only when they are led by an LED light from the back, they should show a particular symbol.

It's a bit like on a laptop keyboard where the letters on the keys have a backlight. But I want the keys to be entirely black when they are not led. The symbols shouldn't be visible at all.