Abrasive materials such as steel wool, fine sandpaper and emery cloth, and abrasive polishing liquids and pastes can work. Liquid chlorine and acidic and base liquids such as "Aluminum Bright" can also dull the finish. Using these products is always risky. Most, such as chlorine, should be diluted prior to use. usually you should apply and immediately remove, then apply again, etc, to make sure the "aging" process is slowly achieved. I've never done it, but I've heard household vinegar, mildly acidic, works, but takes a long time. But, that might be the wiser course to take. I wouldn't jump in with both feet and go all over the blade at first. I would use very small test areas to see what happens. I understand the best way to get desired results is to slightly overly treat the blade and then use a mild polish to slightly brighten it up. Dealing with metal is usually a touchy process. As always, proceed with caution and go slowing, examining the blade as you go.