In my experience, Ballistol can "darken" bluing just a bit. I found this characteristic to be useful when a gunsmith "glare bright blued" a Remington 870 receiver. Repeated applications of Ballistol cut down the glare quite a bit. This characteristic has been less desirable when the maintenance of "bright blue" finishes like those of vintage S&W revovers is an issue. I use other oils on such guns.
I agree that Ballistol is wonderful stuff and I use a lot of it. Its ability to darken blue finishes is about the only characteristic that Ballistol has that I tend to be careful about.
