found this>>The black residue is a very small amount of chromium rust which when it forms a thin coating on stainless steel gives it the brilliant shine. The same effect can be seen much more clearly when cleaning a stainless steel sink using the various cleaners made for the purpose. It can be minimized in cookware by using a suitable cleaner to shine things up once in a while but it has no real concerns otherwise.<<
and then i found this:
http://www.angelfire.com/az2/waterle..._Cleaning.html