If it's as bad as it looks in the pic, the only choice is to refinish the metal, I use automotive crochus cloth that comes in rolls to do the back and forthsies on the barrel with the strips wrapped half around it, makes the work go fast and once it's blued, you can't tell the difference. I completely stripped my .30-30 barrel, then used BC Perma Blue. Of course it's not as nice as a professional hot blue, but didn't cost near as much either, but it still looks great. I've tried doing several barrels just by repairing the bad areas and touch up with cold blue, never was really satisfied with the mismatched blueing.
I trick I learned when browning barrels is to heat the barrel with hot water after you've degreased it, then blue the hot metal with Perma Blue, rinse with water, then card it with #0000 steelwool, then repeat. It usually takes about 4-6 times to get the deepest blue possible with a cold blue.