Browning is a more involved procedure than Blacking. Since the barrels are not boiled or steamed, the rust is "tougher" and requires different carding and "pattern management" techniques, especially during etching procedures. It has a higher potential for fault occurance and thus requires a different "eye" to insure they don't happen. This is one, or possibly the main reason some folks don't offer it.
IMO, it's probably a more durable finish since the Brown form of rust is more stable, and properly done with appropriate contrast it's very beautiful.
Regards
Ken
