I’ll admit my knowledge of metallurgy is informed by a background in welding, but it’s my understanding that the colors on that blade can only happen with a large temperature difference between the middle and its ends, likely as a result of the maker using a benzene torch on it for a few minutes. This high heat is going to do the same thing to the blade as it does during welding: it fucks up the temper. Heat treating is more than just making the metal hot; you have to make it uniformely hot, for a specific amount of time, and then cool it gradually and under control. Doing that doesn’t give you the pretty colors, but it does give you stronger metal.