Guessing it's probably the stuff called sooty mold. That's usually a symptom of a scale or aphid infestation. Both pests secrete sticky honeydew, to which the mold spores stick and are able to multiply. Aphids & scales are often placed on plants on purpose by ants, to be farmed for their honeydew. If this common chain of events is what has happened to your tree, getting rid of the ants should then relieve the pest issue & resulting sooty mold, though it probably won't wash off of this years' leaves. If CM is evergreen in your area, more treatment for the actual mold might be worthwhile/recommended. CM's are deciduous this far north.
