The stone wall in this Sauna almost looks like a waterfall. And properly lit too. –Elämän kirjo: Mun sauna on SunSauna
Untreated sauna benches and panel walls can be blackened using the Supi Sauna Wax by Tikkurila. This water-soluble solution with natural wax is colourless but it can be tinted black.
Another bath inside whower idea: factory window shower - the BEST idea! Imagine this done huge-Japanese-bathroom style with an old claw-foot tub in an oversized shower, so you could have overflowy baths in a shower the size of a small room.