probably black sooty mold. it is a fungus growing on the secretions of insects that might be feeding (sucking) on your camellia. you can treat the shrub with an insecticide to control the pests. i have had the same problem on an evergreen with somewhat similar foliage (michelia) and, once the insects were eradicated, the fungus will eventually be controlled.

Check the underside of the leaves for common tea scale (see attached link). Just as Jeff said, they excrete copious amounts of a sugary substance (plant sugars) that this simple black mold can grow on. It is not a disease. You must control the tea scale for the black sooty mold to disappear.

Check for the insects under the leaf, but if the infestation is bad you can see a mottling on the upper surface from all the damage the insects do.

Here is a link that might be useful: {{gwi:507230}}