There's often some irony involved in collectible weapons.
For example, back in the day when really nice 1903 Springfields, M1 Garands and M1 Carbines would show up, you'd occasionally see a 1903 that had obviously spent a great deal of time in an old school unit between the wars where the stock was hand rubbed with linseed oil once or twice a week and had a spectacular finish. The gun would be 100% correct and original - except for the hand rubbed stock finish.
Collectors would recoil in horror at the "non original" condition, even though it was a 100% normal condition for the issued arm in service, as evidenced by a significant number of vets from that era who were still alive at the time talking about how those fine rifles came about.
It was the difference between a rifle in "factory original" or "never issued" condition versus one that's in a still excellent condition that actually represents how it looked and was used in service.
Let's extend that to a holster, where the collectors are much less appalled. They are less appalled as polishing a holster is a normal part of holster care and does not materially change the "original" condition of the hloster nor count as a restoration.
Here's a 1918 DWM P.08 Luger in very nice condition with a period correct spare magazine and a period correct 1917 stamped German military holster.
In this case there is some polish that's been removed after some moron stuck a price tag on it at some point in the past.
But it's just shoe polish. The rest of the wear on the finish is the normal finish wear you'd see in an issued holster, with some wear on the edges of the flap and on the tip, as well as some fairly normal weathering around the magazine pouch.
Assuming you were issued this pistol and holster in 1941, after it had been used in 1918, and perhaps used again or just stored between the wars, what would you do with it? You'd polish it to prevent getting your head handed to you by someone who expected your leather equipment to look presentable.
In that regard, polishing the holster isn't going to put it in a non standard condition.
Here's the same Luger with the same holster after a polish with the correct color polish. The light is softer and there's no glare as in the earlier picture, but there's no dying involved with the holster.
In both pictures, it is a WWI war German military holster that is worth around $350 holster in this condition, the only real difference is that the "after" polish holster will actually get $350 where the "before" polish holster will not. Set the two side by side with the same price, which one would you buy? The one that looks like it;s been cared fir . Duh.
If you look on the Simpson's limited site you'll see the same holster in similar condition with price tags in the $325-$375 range, even including some where "light dying" is indicated. Polishing to bring a holster back to it's normal condition isn't "dying" and it isn't "restoration", it's just normal holster care and it won't kill the value.
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On the other hand....
You're talking about dying your holster black. The image isn't great but it appears to be a WWI or Weimar era hardshell P.08 holster and virtually all of those were brown. Black started showing up in the German military holsters around 1936 but brown was still the predominant color until about 1941, and by then the strap/stud holster was the norm, not the strap and buckle design.
So, find out for sure what it is and what color it's supposed to be - Dark Brown versus Black - then polish it with an appropriate color polish.
