Why do I have black stuff growing in all three of the toilets in my home?

It's mildew. Kill it with straight bleach or a mildew killer. Have adequate ventilation when using these products.

It is a form of black mold. Where exactly is it growing, in the tank or under the rim of the bowl? What do you use to clean your toilet?

As said above: it is mold. Kill it with Lysol. Also, clean your toilets more often from now on...

My toilets are black from manganese in the well water. Are you on a well? Could also include stinky sulphur smell and rust from iron too.