From Whitney, Naples
The plant is Schefflera arbooricola. The black substance is sooty mold. Certain sucking insects, notably aphids, scales, and whiteflies, suck sugary sap from plants and excrete it as a substance known as honeydew. Sugary, high-energy honeydew attracts certain ants, wasps, bees and flies that feed on it. If these sucking insects are on the plant in large numbers then honeydew accumulates on the leaves and on the soil below the affected plant. Very soon, an airborne fungus, known as sooty mold, also begins feeding and accumulating wherever honeydew is present. Although a fungus, sooty mold is not a plant parasite since it feeds on the excreted honeydew and not on the plant. Honeydew will continue to accumulate as long as the sucking insects continue to feed on their plant host. Sooty mold will persist for some time even if the sucking insects are no longer present. If you treat the plant, you would treat to control the sucking insects and not to control a disease.