Good place to make a slight diversion:
As noted, the Glock Tenifer (or Tennifer) is a surface hardening treatment that is done to the slide before the "colored" finish is added. (See the indented section, below.) The colored finish on a Glock is (or maybe WAS) a form of manganese phosphate (Parkerizing) applied AFTER the tenifer treatment is done.
(When Glock moved production of guns sold in the U.S. market to the U.S., they were forced to change the Tenifer formula. As it is done done in Europe it can be an environmental hazard, poisonous for those doing the work.
The complaints you hear about ****** Glock finishes with the new guns (made in the USA) has NOTHING to do with the changed Tenifer formula, but with changes to the colored finish applied afterwards. Glock has apparently changed THAT a number of times.
S&W apparently uses a surface hardening treatment very similar to Melonite (which is a proprietary name S&W can't use.) The colored part of the finish is apparently a layer of something applied AFTER the slide is hardened.
From Wikipedia:
Ferritic nitrocarburizing, also known by the proprietary names Tennifer/Tenifer and Melonite, is a range of proprietary case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures during a salt bath
Someone recommended using Flitz... I'd avoid that, as that could make the "scratched" area shiney. (That said, there is a chemical component to Flitz that that COULD do some cleaning.)
Instead, I'd try
very lightly rubbing the "scratch" with one of those 3M "Scotch Brite" pads (or one of the generic versions available in most grocery stores -- very cheap). That would likely take off anything deposited there (residue) and NOT make the result shinier that it was before it was rubbed.
If that doesn't work, S&W may have some touch-up paint, somewhere, too. If that's thinned a bit and applied delicately with a model paint brush you'd probably never be able to tell it was repaired.
You could probably get some model paints and mix up some yourself to fix the problem area, if scuffing or buffing doesn't fix it. I've done this with a couple of my guns that got "ouchies" and if it's thinned down slightly (so that it doesn't get thick, its hard to tell it was "repaired."
Start with a matte black and matte white paint and mix up some to try to get a match or the right shade of black or gray. Mix up tiny quantities.