Interesting, purple tulip - and nice observation.
Although I'm w/ cd - I don't think it's the dead spirochetes.
I live in a very humid climate and my bathroom is generally a moldy haven. I have to leave the exterior window
open all the time, which really helps. I also have to leave the toilet seat cover up so that the bowl airs out constantly and I have to use a bleach disc in the tank (this is NOT good for the environment but that has to take a back seat to my mold issues for the time being)... otherwise, I get the black ring.
I know the black is mold in my toilet... might be mold in your toilet too, or something else. We do excrete a lot of biological junk. But since it's mold, every time the toilet flushes or the mold is splashed, there's the risk of mold spores being released and none of it is a good thing. You really want to eliminate mold from all surfaces as we're struggling through treatment. Bleach is the fastest way but it's not necessarily safe or good for the environment. But it's what I use in the toilet area. The rest of the bathroom I clean with vinegar.
ALSO... do you have any yeast/fungal issues? This also creates a moldy environment in the toilet.
I came across a great video showing you how to easily clean your toilet. I've kind of been doing this except I've been using one of those long-handled brushes, which I now realize is probably "spraying" germs everywhere. So I'm going to use a scrub/sponge next time, probably with better results.
www.huffingtonpost.com/entry/how-often-should-you-clean-your-toilet_us_5722463fe4b0b49df6aa7f2b-p