It sounds like sooty mold, in which case you need to control the insects that are producing the honeydew that the mold grows on. Depending on which pest it is you can use a soap spray (preferably insecticidal soap made especially for plants because it's less likely to cause injury, but you can use a homemade soap spray) or an oil treatment. This article makes some suggestions for appropriate commercial oils but if you prefer home-made I'm sure someone can help with recipes. Do you know what pest is causing this?
Http://baker.ifas.ufl.edu/Hort...
