I've done a fair bit of chimney sweeping (and might have to again the way things are) and would just say that it is fairly normal to get soot settling on the stone but believe me if it has only settled there then no worry just vac it off (never ever wet it without knowing what your doing). I'd be more concerned about the skirting boards nearby!!
Sometimes there are difficulties masking an area off but with stone there is no fear of ruining anything so I agree with Kevin, complain to the Sweep who should cleaned up after himself anyway.
ps how much did he charge?