Blueing is a special form of rust
And the exposed surfaces of a gun will fade an change color over time. Modern techniques do it less, and of course, more modern guns have not had as much time pass either.
Look at a lot of old guns and you see the blueing has turned brown (to some degree), and really old guns were "browned" as their original finish, before rust blueing was understood and widely used. The "Brown Bess" British musket is a famous example of browning as a gun finish.
Put your blued gun in a nice safe for a hundred years and you probably won't notice much change in the color of the blueing. Exposure to light, air, human hands, weather, rubbing, different solvents and oils, and everything else you can think of over the life of a used gun can, and does have an effect on the finish.