Bluing repair

I have not found a product that would match the blued finish over a significant area. I have used the Casey,s cold blue to touch up minor defects at best several mm in length and a few mm in width or traces of blue wear on the front of a barrel with limited success.

Brownells sells some very good products for cold bluing.

The one ive had best luck with is Oxpho cold blue . The gel is easier to work with ..Just warm the metal and wipe on / wipe off . 
ive used it on touch up and blued complete rifles with it . 

Listen close,,,I don't jump on these post with advise,,,,there are two products by this company,one for wood,and one for gun blue,,,I build traditional bows long bows self bow what ever ,and I refinish bows ,,,,,my got to product is "Vans instant gun blue " ,,,and his "Arrow wood Finnish " I met van at a gun show,,,he stuck a wood gun stock in my hand,,,, (just the wood) made me scratch it and he fixed it right before my eyes,,,then he taught me how,,,,fantastic stuff great guy I use a lot of this stuff,,,can't say enough just love it he has lots on the web! I just re blued an H&R handi rifle the blue came out awesome can't wait to mess this .357 max up this deer season sorry for the rant ,,,,,JeffW