Aragorn, I'm not sure how much of your spear would have originally had a bright shiny finish and how much would have originally been a blackened finish. Many years ago I did do a little bit of blacksmithing for a few years and one of the finishes that I used, strange as it may sound, was to apply black boot polish to the forged metal while it was still slightly warm. While I'm not suggesting that you or anyone else heat up your/their spear/other object I do believe that black boot polish was quite commonly used to give a blackened finish on forged metal.