The black finish isn't Tenifer; it's Parkerizing. The Tenifer finish is applied to the metal in a 900 degree Centigrade nitrate bath, and is impregnated into the slide.
There are a few outfits that'll remove the Parkerizing from a Glock slide. I don't know why folks want a shiny Glock, but that's the glory of a capitalist society–if someone's willing to pay for it, someone else will do it.