I took a closer look at your photo, and your description was right, it looks like it "came off" and did not corrode. If it is that new take it back to the dealer, there should be a warranty on workmanship. Some companies are going to a hot water submersion set up to blue, so they can beat working with caustic chemicals. I used to do a lot of rust bluing, which is a long the same line, and occasionally would get thin spots. It kind of looked like that after a while. A lot of things affect the bluing when rust bluing. Could be the area was not super clean, minerals in the water, impurities in the steel, and things for no reason. If you did not abuse it, I would try taking it back.