Remember that rust blue is a chemical process which can be activated, or de-activated at will. Abrassion is the usual cause of bluing loss at the muzzle. My 20's H&H has about 40% original barrel blue remaining and I have not re-blued it. That is just me, many others would have long ago. If your main concern is the localized blue loss but you do not want to polish and blue the whole barrel set you can blend out the area of loss only.
De-grease the entire barrel set but only apply your rusting solution to area of loss, boil and repeat 6-8 times or until the color is a close match. A few passes over everything will then blend things nicely usually. If not repeate as needed.
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