I'm operating on the assumption that the finish is rubbed off or never made it onto the metal there, and he is looking for corrosion protection as will as to reduce the contrast. I did not recommend most cold blues as I've seen a number of them actually encourage rust. I can't spot my photo just now, but I applied a wide variety of them to music wire rods one time and rinsed them and left them un-oiled to sit around the house for a month in summer when humidity is higher than in winter and after a month, all but two had after-rust. One very badly. That happens if you don't get the acid base chemically neutralized or boil it in distilled water as you would for rust bluing, then apply water displacing oil. The rust blue I mentioned will largely prevent rust if it is kept oiled and it is more durable than any cold blues, but you do have to go to some bother to put it on. Oxpho blue and Van's are the only two cold blues that didn't encourage after-rust in my experiment. That's because they are phosphoric acid-based and the coating they produce is like micro-phospating—much thinner than proper arms phosphatizing, like Parkerizing, but still tending to discourage rust. The color is not really blue, though, and not very dark. More like charcoal grey.
If he is looking for a true color match, though, and not just corrosion resistance and partial camouflage of the bare metal, then he's got a more severe set of constraints. The best option for a true match is usually going to be returning the part to the maker for refinishing.
