Encountered already a few times online some Tsuba with a weird blackened color...wondered what happened because some looked like real old tsuba.
Some seem to put color enhancing rust prevention product on their Tsuba???
Some sellers even sell this stuff in Japan:
Sword accessories (tsuba and iron accessories), Nambu ironware, iron kettles, antique ironware, natural "white glue wood condensed tannin" color enhancing and rust prevention agent, turns red rust into black rust (tannin iron) | Jauce Shopping Service | Yahoo Japan Auctions. eBay Japan
Not a good idea if you ask me?