The simple physical act of rebluing has little to do with ethics.

It is what happens to, or is done with, the rifle afterwards, where the ethics issue arises. Since we cannot take our toys to the afterlife, the "personal property - I'll do what I f-well please" argument holds no water, at all.

If the arm is in 100% military configuration, but in less than pristine condition, then I think most people would agree that it should be left alone.

If it has been brutally bubbacized, then I would think that few people would care.