Good stain blocker should seal that contaminated area. However, if it is only this one sooty spot on the wall / ceiling corresponding to the tree on exterior, I would also check your exterior wall finish condition, flashing and roof/gutter/attic section directly above this spot.
There must be a reason for the soot accumulating in this particular area – make sure that it is actually soot / not some king of a mold caused by excessive moisture. Shade / the tree could be responsible for trapping moisture in this area / slowing evaporation, but the moisture must penetrate the wall somehow.
There is more about it here: stains on walls