I've never tried darkening glass, but I guess just going over it with whatever color stain you want will do the trick. You have to be down to bare glass for it to get in there though.Itshouldn't soak in very deep at all, so the color will just be right on the surface. I wouldn't do much more of a buffing to the stainthan just a light going over with 0000 steel wool.

For all I know, it might not work at all, but it's the only thing I can think of to try.