Breakfree was not originally intended as a general lubricant, but as a penetrant to free up rusted or frozen parts (hence the name). I don't know if it would harm rust blue or not, but rust blue is definitely rust and can certainly be removed by rust removers. (I used to remove it that way instead of polishing to preserve the markings and corners on guns like the Luger.) I think I would go easy on the Breakfree and use another cleaner and a good gun oil for lubrication.

As to Remington using rust blue that late, I am a little surprised. I have Remington guns from the 1950's and they are definitely tank blued.

Jim