Rancher: There can be a number of explanation for " spots " in the finish. The one I experienced was slag in the metal that was not made of steel, so that I had to heat that area up with a torch to get it to brown like the rest of the barrel.
Another reason for spots is that you didn't get the surface cleaned enough before bluing it. You need to use a very good solvent and cleaner, like acetone, to clean the barrel before bluing, or you can get bare spots, or lighter spots. Alcohol cuts through must oils and greases. But unless you are using denatured alcohol, the alcohol has additives that will remain on the barrel. That is why rubbing alcohol and isopropyl alcohol are not always your best choice for this kind of work. Now, if you remove oils and fingerprints by using a good hot water bath with detergent, and then rince the barrel of the detergent complete, and then use alcohol to clean any possible remnants of oil, grease, or detergent, you can usually get a satisfactory finish. But, you will need to heat the part to be blued up so that water boils on contact with it, to open the pores enough to get out all the oil and grease that may be on the surface.
Hot bluing involves several tanks, only one of which holds the bluing salts. The others are for cleaning the barrel completely, while hot, ( boiling them clean is a good description), and cleaning the salts off the barrel AFTER bluing. With four " tanks " in most set-ups, only the third tank contains the bluing salts. All have gas burners under them to heat them to boiling. When you do cold bluing or browning at home, you still have to take the care to clean all the parts completely to get a good result. If the part with the spots is small enough, boil some water on the stove in a large pan, and dip the part into the boiling water for a couple of minutes. When it comes out steaming, its ready to blue again. A second bluing should remove the spots.