As Haas said above, it's very difficult to scratch a Glock deep enough to go through the Tenifer. However, the blue or parkerized finish is applied on top of the Tenifer. What you are seeing is a scratch in the blue/parkerized surface finish, exposing the base metal, but that metal is still protected by the Tenifer, unless it was a very deep and seriously abrasive scratch (like from a file or grinding stone). Some owners actually polish away the exterior finish on purpose, leaving the slide with a textured bare-metal look, and their Glocks STILL don't rust, because of the Tenifer treatment.
You might try a touch-up bluing chemical to make the scratches less noticeable, but most chemical "cold blue" finish formulas can be described as a form of chemically controlled rusting, so they may not work well on a Glock's treated metal parts.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)