Tennifer finish is actually not a finish at all, but a metallurgical process performed on the metal parts of the Glock pistols and knives. The metal is treated with a cyanide mixture bath to increase hardness and corrosion resistance. This is not a finish but is actually impregnated into the first few microns of the metal. The Tennifer treatment is colorless.
Glock simply powder coats the slides black after the Tennifer process is complete. You can take a piece of sandpaper to your slide and remove all of the powder coat, leaving your slide a dull stainless color (which looks pretty cool) and the Tennifer properties will be left intact.