I had exactly the same thing on mine and on inspecting the leaves underneath I found it was due to scale insect, little brown bugs that cling on like limpets. I had to wash every leaf individually (did it outside with slightly soapy water and cloth, then rinsed off with the hose) . They are stubborn little devils and you really have to rub to get them off, although the soot comes off quite easily and the leaves were glossy green under the soot. After that, I sprayed it with bug clear. It was fine last year, but I shall be inspecting it again at the first sign of the black stuff. Good luck.