As was said, just about any weak acidic solution will do the job. I've done a couple of blades now (the blades sorta sucked but the patina was wonderful). I used warm vinegar. Clean the metal (did we say that already...???) and drop the blade into a solution of household vinegar that has been warmed in a microwave. Do it outside. It will start to bubble. Wait five minutes, take it out and wash it off (don't rub it or, if you do, rub it with a very soft cloth). Dip it again for another five minutes in warm solution. You will get a very nice patina. Protect the parts you don't want affected with finger nail polish or other lacquer. I protect the edge on mine and then hit it again on a polishing stone or lapping film.
Here is an example. Blade sucked but the patina was nice.
Inventor of the world's first safety vibrating Kamisori with night light. Go to http://badgerandblade.com/vb/showthread.php/299465-A-milder-Kamisori: Inventor of the Weckisori - (thanks sychodelix)