Sooty mould growing on the honeydew left by sap-sucking bugs such as aphids or scale.
The mould can be washed off with the hosepipe - the bugs need killing. You can go the toxic route with branded chemicals (but not anything containing neonicitinoids) or the environmentally-friendly way - a cup of 100% vegetable oil mixed with a tablespoon of 100% soapflakes (NOT detergent), diluted into 1 gallon of water and sprayed on, making sure you spray the underside of the leaves.

Most likely you have Cushion Scale infestation - check beneath the leaves - you're looking for tiny, yellowish looking things, usually sitting near the central vein of the leaf. Later, you should see white, fluffy, elongated egg cases. Spray treat in late June/early July with an appropriate insecticide. Google Cushion Scale and select the RHS page for more information and recommended treatment products.