So then, assuming the hatta is ally, then anodised, you want to sit it in something that will make it go black? (some kind of acid/chemical?) am I on the right train of thought?
If its painted steel, all you can do is burn it or paint it.
there may be a process to turn anno'd ally black, but i'd ask your local electroplaters for more `on target' advice!
neat chlorine (about 80%, tesco stripy bleach is about 1%) turned my brass plughole black, might eat the ally though!
Mark M