I have been having black streaks in my toilet bowls in my new home--2 toilets, same age/manufacturer, and I see black coming out from the holes under the rim. After a little investigating, there are two bolts or screws at the base of the toilet tank. They are completely rusted out and disintegrating. I scratched one of them, and black sediment came off it. I'm pretty sure I figured out the issue, and hope this will help other people. Attached are some photographs. The screw on the left, below the overflow tube, has not been scratched or rubbed, and you can clearly see how rusted it is.
I wonder if my property inspection companies should have found this, and I wonder whether my 1st year home warranty will cover this. Even still, there is a $100 copay per toilet. It sucks. But I guess that would be cheaper than having a plumber do it, right?
In the attached pictures, the screw on the right has been scratched and rub, where the black sediment is, but the screw on the left is still rusted in just like the one on the right was before I altered it. What are those screws anyway?