I have a bluish-gray "ring" that develops in my toilet bowls, more evident towards the upper area of the water level.
It really doesn't come off. It feels like a stain within the porcelain, as there is no crust, etc. Vinegar or bleach have no effect. I tried scrubbing the ring with Fast Orange with pumice, and it did notably remove some of bluing, but not all.
There is no blue coloring used in the water, which comes from a well, through a softener I got from Speedbump (Fleck and Clack, Resintech beads, rock salt), but I think this was starting before I added the softener.
Photo shows it with the water level down before I started cleaning it.