Sooty mould on the leaves is caused by scale insects on the underside of leaves. These look like pale brown pinheads and are hard to spot as they are flat and live close to the leaf midrib.
Sticky honeydew drips from them onto the leaf below, where it is attacked by a mould, which turns it black. In May, they leave a trail of white fluff full of eggs a few millimetres long.
They are more of a problem in conservatory/greenhouse conditions. Out of doors, the rain washes off the sooty mould and predators control the insects.
Spray with insecticide or rub off with your fingernail; the sooty mould washes off with a spray of water or you can wipe the leaves.