Sounds like sooty mold, Bar, which usually forms when you have an infestation of greenfly. Give your skimmia a good, strong spray with the hose, on the undersides of the leaves if you can, and do it again after a few days. You can use insecticide, but this would kill off the GOOD insects as well.
You could try looking for some ladybirds to put on your bushes, too, because they would get rid of the greenfly for you, too!! Good luck, Annie (Cumbria)