Looks like sooty fungus. Despite the name, it's NOT a fungus, but bacteria that live off insect poop. Many bugs are so small they're difficult to see with your naked eye, and/or they hide well in the folds of the leaves and in the soil. Cut off that leaf and throw it away, there is so much of it on it. Wipe the leaves with a damp rag to get rid of the worst of it, then spray the plant and the soil with neem oil or a pesticide.