I don't know of anything that won't hurt blued finishes but if you go to your local auto parts store you can find solvents that remove the blue and yellow marks that form on exhaust pipes on motorcycles. It should work on Stainless revolvers because it is the same process! There is a link to a website for a bottle of blue away that does this:
http://www.hmcycle.com/webstore/viewite ... e=viewprod
http://www.hmcycle.com/webstore/viewite ... e=viewprod