what to do about it
It isn't like the powdery black problem we've had before and seems to be only on the one plant. There are five camellias in that area. It seems to be right through the leaf as though it's been scorched but it's badly affected.
Did you fertilize the Camellias? Fertilizer burn can appear as scorched leaves.
I have listed a couple of links with more information on the common pest and fungus issues with Camellias.
If you don't feel it's a fertilizer issue, I would treat with Neem Oil. This acts as both a insecticide and fungicide.