The black could be from the deteriorating rubber componet. While your in there and have the tank clean...closely inspect the flapper, rubber washers that hold the tank to the base, and fill valve replacing any/all that need it. I doubt you'll even need any bleach...just a rag and clean water. I assume you're on city water? If not...I wouldn't want a large volume of bleach going into the septic system.
when it comes to breakfast, the chicken is dedicated, the pig is committed.