When a gun is blued the bluing salts have to be neutralized after the color is attained or they will continue to oxidize (rust). Water is often used with cold blue, but with the hot blue the water/baking soda would probably be better. Could be the salts weren't 100% neutralized when the factory blued the barrel. I'd try the water/baking soda and see if it stops the reaction.
(If it were mine I'd soak it in a utility tub or bathtub with hot water & baking soda for an hour or so.)