>> I don't want to do anything too drastic and take off the patina/color of the rock
Someone may have a better idea, but I can't think of anything that would take off scorch marks other than abrasives. You could try Comet cleanser with something small, like a small wire brush or a flat pebble, and try to clean only the scorched parts.
But those parts will still be scratched up, plus scratches where you overshot. Might as well use something like sandpaper on a fingertip, a Dremel tool or a small grinder wheel on a hand drill.
BUT, after you scrape or grind away the scorches,
paint the raw rock surface with a little buttermilk or plain yogurt (or beer?) into which you have ground up some lichens or moss of contrasting or complementary color and texture. That's supposed to be a way to "seed" rocks with interesting lichens or moss.
Moss grows much faster. I speculate that you might get some moss within a month or two by keeping the rocks moist and shaded. Once they're presentable, you could move them to a drier, sunnier spot and see if the lichens take over from the moss. But I read that lichens only spread by 1-5 mm per year! You might try to collect several kinds of lichen so you get at least some compatible varieties.
Grind the lichens or moss finely, or run them with the liquid through a blender (try for a thick slurry, not runny or drippy. One site called the best texture "creamy and spreadable". One site suggested thickening it with corn syrup.
You can paint it on, smear it on, blot it, dip it or finger paint it.
Perhaps the new lichens would grow OVER scorch marks, and eventually hide them without any grinding? Tentatively, from reading, moss likes to have scratches or a rough or porous surface so it can get a grip.
http://vivariumguide.com/grow-...
http://guerrillagardening.org/...
