Long as there's no rust, leave 'em alone. Discoloration/darkening is a characteristic of carbon steel(non-stainless). Can't really clean that away without removing steel from your blade which will definitely hurt the value and not prevent a return of the discoloration if you use it at all. Light rust can be removed with WD-40 and OOOO steel wool if need be. Don't use a lot of presure, just enough to remove the rust. Keep it clean and dry, and coated with a rust preventative, and appreciate it as it is; which I'm sure you do already.
