how to "smoke" clear plastic?

I have a clear plastic shell.  I don't know exactly what sort of plastic 
it is, but it's probably not polycarbonate; it was cheap (bought it at 
the dollar store).

I need to paint this with something that will make it look like smoked 
plastic.  Some sort of translucent black paint?  I will have lights 
(LEDs) inside, and the goal is that these lights should show up clearly 
when lit, but be completely invisible when dark.

But dammit Jim (*), I'm a roboticist, not a modeler.  I haven't painted 
any plastic since I was a kid.  So I'm hoping some kind soul here can 
recommend a specific paint and application technique that is most likely 
to work, even in the hands of a newbie such as myself.

Thanks,
- Joe

(*) That's a Star Trek (original series) reference, for the folks here 
too young (or insufficiently geeky) to remember!