Check the undersides of the leaves for scale insect or other infestation - scale produces honeydew and sooty mould sets in on the upper surface of the leaves as a result. If its got scale (if its obvious, just shows how mild the weather is currently) then treat for that - the sooty mould should just disappear.

also greenfly leave a sticky deposit [honeydew] that can encourage sooty mould. If its a small shrub you can wipe the leaves to clean it off the surface.