Answer: The black film is likely a sooty mold that grows on the honeydew produced by aphids. As aphids feed they excrete a sticky substance calley honeydew which turns black as it ages. To control the problem, you need to control the aphids. You can spray with insecticidal soap to get rid of the aphids, and this may wash off the sticky coating and the mold as well. If not, you may need to wipe each leaf with soapy water.
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