The leaves on my mum’s camellias developed a nasty bout of black sticky soot. This eventually killed the leaves, and the flowers were not so good, either. The cause is the scale insect. It looks like this. The RHS says this about it.
Following various bits of web advice, my mum tried swooshing the plants down with a hose and spraying them with whatever proprietory insecticide she bought for the purpose. Neither worked. I have just spent three hours painstakingly wiping each leaf, and the stems, free (I hope) of scale insects. I don’t think there is really any alternative.