Check the branches, stems and backs of leaves for signs of scale infestation - these look like little bumps on the stem, sometimes with white fluff, sometimes not. The sooty mould is often caused because of the presence of this pest. Provado make a treatment for scale is that's what it is. Otherwise, sooty mould can be caused by aphid infestation, so check for that too. This assumes your camellia is not directly under a tree which may be infested with either of the above mentioned pests.

My brother had a good sized Camellia in his front garden that got infested and looked sorry for itself. He did a drastic prune, and fed it well, and it is now in fine form and as good as ever. Took about 2 years to get back on track. Prune after flower time is over, and keep well fed and watered in July/august.