Not knowing the history of the barrel you're having the problem with, I can only relate an experience I had. A similar problem when rust blueing a set of bbls had me wondering what I was doing wrong finally explained itself when the owner told me that the small area in question has been worked over by TIG welding. Apparently some pitting in that area had been cleaned by bead blasting (which can in itself also cause blueing problems) and then the pits TIG welded. A nice job of welding from a cosmetic standpoint but caused alot of blueing problems. Changing to different blueing solutions I finally found a quick rust that covered it with the rest of the barrel.
FWIW..
Laurel Mtn for slow rust and Mark Lee's solution for quick rust blueing have given me fine results over the years. The L/Mtn definetly needs to be watched for after rust and a good soaking in soda solution helps kill it after the process is done. Sometimes I put a coat of L/Mtn on a barrel first and then finish up with quick rust blue. The L/M etches a little especially if you let it sit so you have to watch it carefully. I think it has Ferric Chloride in it. Works OK too in lower temps and humidity, just takes longer which is OK 'cause you can control it better. Putting a quick rust blue soution onto the metal with a clean steelwool pad instead of a cloth pad seems to work better for me also. Scrub in the first coat good onto the hot barrel. I polish to what the original gun was in a restoration. Outside of that my usual polish is 400 but high polish rust blue is simple enough, just more time consuming. Weaker solutions, shorter rusting times and more cycles if using slow rust. Easier to keep the high polish with quick rust process. Alter the carding technique for both to preserve the polish of the steel. I won't use the 'new' Belgian Blue from Brownells as I believe it has the same mercury compound in it as the original and I probably used too much of that in years past. The new stuff will show the same telltale silvery signiture when you swipe some onto a piece of warm brass or copper. Messes up any gold inlays on a piece being blued anyway,,unless you like them all looking silvery..
