What is this black stuff that appeared on my drain after cleaning with a bleach-based cleaner and how do I clean it?
This may not be mold but the chrome plating having pin holes and the cleaners / moisture eating the base metals have created a new problem. I would try to dry the area and use a quality fingernail polish to seal them. If this works it may last for quite a while but with plated fixtures once moisture gets under the plateing it is hard to stop the damage.