I've only "blackened" stainless with two finishes. The first finish I can't do at home and had to send it out, that was melonite.
The second finish I do at home and it's very durable. I use molybdenum. I get it from John Norrels, his company calls it moly resin. You need an oven long enough to fit your barrel and a gunsmithing thermometer, since the one on your oven isn't accurate enough. And don't do it when your wife is home, otherwise she'll complain about the smell.
The surface has to me slightly roughened with some sand blasting. If perfectly smooth with bead blasting or polishing, the molybdenum finish won't be durable. I found it works best with stainless but also good with regular steel and it is least durable on aluminum. My guess is that it works better with metal that has longer heat retention, but I'm not a chemist or metallurgist, so that's just a guess
