It looks like mold or mildew. That's environmental usually brought on my not enough sun, poor air circulation & excessive moisture. Can you move it to open space in the sun, prune out the old branches or cut away surrounding shrubs.

That is sooty mould. It grows on the honeydew excreted by aphids. So killing the aphids is a good first step, the next thing to do is to spend a few hours wiping the mould off the leaves. Try spraying with a fungicide too.

Is the shrub by any chance overhung by a sycamore? it happens a lot with them.