I would have to ask how a particular part was blacked out first. They are not all done the same way or with the same type of finish. Never had a blacked out ND hub in my hands but looking at all those in pictures it appears the shells and brake arms are coated with some type of paint. On the Schwinn hardware I have it appears to be blued. A year ago I refurbished one of our machines that went thru those Texas floods about 2 years ago and that machine had some blacked out hardware that was a hard gloss black and appeared to be some type of bluing. I used a metal prep that's used in the body shops. Phosphoric acid. I used it straight out of the jug uncut. Brushed a good coat on just the rusted areas and let it sit for about an hour and then rinsed it off. Where the rust was I had clean new looking metal and it did not phase the black finish. The parts were the pressure plates used in a stamping machine so they were undoubtedly heat treated and they may have had some bluing formula on them before they were thrown in the oven.