Anything you rub a blued finish against hundreds or thousands of time is going to wear it. A hard Kydex holster actually touches the gun only a few places and will put narrow streaks and spots of wear in the blue. A leather holster kind of massages it all over as you holster and draw and causes an overall softening of the blue, starting at corners and edges, and eventually turns large areas gray.
Fobus is junk, molded and riveted, I have had them that were ok, but the next was too tight for a smooth draw.
Nylon holsters are trash, I have never seen one that is really suitable for serious use.
Sorry, Link, but a gun is a tool and really using it means putting some wear on it. Probably the gentlest would be a Safariland with composite construction, plastic to hold shape and a suede lining against the gun. But you have to keep the lining clean or it will pick up dirt that will grind the finish.