Dip it in ferric chloride is most common I think. It's available at some Radio Shacks as circuit board etchant or in powder form from on line suppliers. Mix 3 parts distilled water to 1 part ferric. Soak the blade for a couple minutes at a time, wipe it down (not aggessively), soak again until you get the look you want. When it's done...I spray it down with Windex to neutralise the etchant. Rubber gloves are a must and don't leave the open container in your shop for long periods of time, the fumes will rust everything and they just aren't good for you. Use the search function on any search engine, you'll find out more than you wanted to know about it. Be safe.
Rudy