My acupuncturist said that the black tongue might be due to problems with my waterworks too but I've read several times that it is due to an imbalance in bacteria and/or signs of a fungal infection.
I've read about liquid gentian violet, I will see if I can get this. The white covering isn't really bad, it's just at the back which needs clearing. I get awful chapped lips but I think the oral nystatin is helping with that.
I think I need to look into biofilms a lot more like you did.