Hi Elaine, Sounds to me like it's covered in 'sooty mould' and I'm sure at some time you have had aphids feeding on this plant ? This sooty mould feeds on the residue left by the feeding aphids. You can also get this if your camellia is under trees, especially Lime trees, as the leaves release what is almost a sugary 'mist' as they photosynthesise and the mould grows on this, thats why cars etc. also go black when parked under trees (especially Lime trees)
Have a look at the new growths and I bet they are covered in aphids, in which case you need to spray with a systemic insecticide ! Once the aphids are gone, you can then wash this sooty mould off. It doesnt do any dammage to the plant other than rendering the less less efficient at making food by physically blocking out the sunlight from the leaf but, it does make the plant look unattractive.
Lannerman