Can taking ferrous sulfate supplements cause dark green/black stools?

Oral iron : Yes, for sure. Famous and familiar side effect. Our intestines can absorb at most 20 percent of iron from a tablet. 80-90 percent will pass mixed with stool and mucus.

Possibly: I'm aware that Iron digestion causes dark black or green stools. Chemical digestion is responsible for this. I would continue the Iron and inform your doctor in case they would like to perform tests.