What is the black film I have been noticing on the leaves of my shrubs?
This is sooty mold, which is a fungus that is caused by the excretions of honeydew from insects like aphids, scale, greenbugs, and mealybugs that in turn attracts ants which encourages growth of the fungus. The cure for this is to treat for the honeydew producing insects that are ultimately the cause of the problem. Once treated, this will stop the problem from spreading and new growth that emerges from the plant will not be infected by the black fungus. However, soap, water, and scrubbing may be needed to remove the film from the old foliage.