It could be mold from the toilet too.
Or the rubber flapper may be desolving from the chemicals in the tank.
Mold is a spore.
Where the drywall is moldy, it will need to be cut out and replaced after the shower is fixed.
We have a 20 yr old, two story home with a master potty that leaks black stuff when you flush. It's awful and is very hard to wash off. It seems to be coming from the back of the tank, even with cleaners put in the back of the commode, it still has black stuff coming out. Also, the upstairs sinks have backed up once with sewage. Could this be a backed up system or a pipe installed improperly? I've had a plumber in and he didn't seem to think it was a big deal. It's just embarrassing, it always looks dirty. If you have any solutions it would be greatly appreciated.
PS: This home also has a shower that leaks into the downstairs, if anyone has any solutions to that one too, the bathroom reaks of mold.
Try pouring a cup of bleach in the tank and let it sit over night. The next day take a new bowl brush and scrub the tank out well and rebuild the tank with a new flush valve and flapper. It may help some. If the black is in the bowl portion there may not be much you can do short of replacing it. How old is the toilet and does it flush normaly?