You have to prevent the disease by spraying beginning in the spring. No it didn't, because the problem is caused by aphids feeding in the trees above the rhodo. They're sucking insects and the sap drips down on objects below. A black fungus called sooty mold grows on the sap. You can eliminate the black by controlling the aphids with a labeled insecticide.
Fungicides have no effect on aphids.
Every year my marigolds look fine when I plant them but then the leaves get white and speckled and the plants look sick. Don't know about the soil, but the marigolds are covered with mites. Spray them with insecticidal soap, horticultural oil, neem, or a miticide such as Kelthane, being sure to get the bottoms of the leaves covered.