A Google search on "sooty mold" turned up many photos of affected plants, but only you could say whether these plants appear to be affected like yours are.
hiya yep it is most proberbly is sooty mould as it can be a problem for a lot of trees and does make them look black you should be able to rub it of in your finger tips. Its caused by aphids ect, excreting honeydew which is what the mould grows on ,so best cure is deal with the aphids ect
