Stainless blades won't 'patina' (darken or visibly oxidize), per se. But, as with any blade that gets used, the usual scratches, scuffs and discoloration by acids or whatever will all leave a 'history' of use on the steel. If left unused, an old stainless knife in new condition will continue to look like that indefinitely (assuming it's otherwise protected & cared for).
David