Question
Camellia leaves turning black thick coating can this be treated

Answer
I suspect your camellia has some type of insect problem.  Several types of insects (Aphids, Whiteflies, Scales, Leafhoppers) feed on leaves and exude a sticky substance called honeydew.  The black coating you are observing is the result of sooty mold, a fungus that feeds on the honeydew produced by the insects.  The mold itself will not kill a camellia, but it can cause stunting and reduce vigor since it interferes with photosynthesis.

Sooty mold does not generally penetrate the plant, so you may be able to wash the mold off or wipe it off the leaf surface.  To completely solve the problem, you need to solve the insect problem. Start by washing the leaves including the undersides to remove/dislodge insects.  You may need to treat with a horticultural oil or insecticidal soap to completely eliminate the insects.  You should be able to buy these products at your local garden supply store.