Is black mold dangerous?

Dark color does not identify a species and it does not, by itself, prove a medical emergency. Stachybotrys is one dark mold that grows on chronically wet paper and drywall. Other common molds are also black. Any of them can bother people who are sensitive, especially in a damp room, and any of them should be removed from building materials rather than painted.

We cannot tell you that a stain caused a specific illness. If someone is sick, talk to a doctor. If the wall is wet, fix the wall. Do not disturb the growth to 'see if it smears' while the furnace runs.