Mold ring in toilet bowl

I have a black mold ring in our guest bathroom toilet that doesn't seem to want to go away. I've used bleach, toilet bowl cleaner, etc. The top mold comes off when clean it, but them there's almost a black mold stain? Could the mold be in the porcelain somehow? Idk, but does anyone have any tips? I keep my apartment very clean and the mold ring in the guest bathroom makes me a bit self conscious when guests use it.