I hope you are not going to paint it now. But either way you need to look at why the soot got there in the first place. With older houses that leaked air like mad, there was lot,s of replacement air to feed the fire, of coarse that was all those drafts that we didn't like. After years of trying to tighten up houses even older houses have tightened up and now we have close dryers, bathroom and hood fans and maybe furnace all looking for replacement air. And sometimes just poor fireplace design and sometimes a decritive sheet metal flashing across the top of the opening will help.
Sometimes it's just how the fire is started and insuring the draft before droping the flame on the kindling.