I'd sure be suspicious of sooty mold, and it's a secondary effect of an insect infestation. Have you seen any insect pressure at all from maybe mealybugs, aphids or scale? If it is black sooty mold, it's not harmful of itself and grows on honeydew.

Sooty mold was my first thought, too.

I would spray the plants with a strong stream of water to help remove mold and insects. But not too strong as to damage the plant.