Hmmm I'm wondering if waterless hand cleaner will work? GO-JO or something like that...
Take a scrub brush, and a liberal amount on it, then scrub the area and wash off with some water from a large sprayer...(hose type, not blast with a power washer... I would worry about damaging the shingles with that).
Since its a petroleum product, it should cut the soot, as it would your hands, and be gentle to the shingles... (but as always, test on a part of the roof tha isnt going to be showing or spare shingles..)..
I think this might actually work...to think more about it