You can coat the brass jag with Vaseline lightly and then run a soaked patch. If no blue then it's the jag. I use nickel plated jags.
Nickel is pretty hard, I don't think that I would want that coming into contact with the lands of my barrels.
I make jags out of Delrin for each of the calibers I shoot. Gets rid of the false blue, are safe for the bore, and I can tweak the diameter to get just the right fit for the patches I use.
