black film on plants

[Clayjacks]

It's also called sooty mold. It grows on honeydew (read: excretia) from, most commonly, aphids, scale and mealybugs.
The mold itself isn't harmful unless it's really bad and interfering with transpiration or photosynthesis- in which case your plant is already doomed from the sucking insects. Try a hort oil, or soap, preferably on a cool(ish) or cloudy day, and focus on the insects rather than the mold.