Without a photo, I'm going to go for the most likely "usual suspect": sooty mold.
If you've had aphid, scale, or mealybug attacks on this camellia, sooty mold is a likely follow-up ailment. My Organic Gardener's Handbook of Natural Insect and Disease Control
(Ellis & Bradley) recommends using warm soapy water on the leaves and then thorough rinsing.
Depending on how large the plant is and how recently you might have applied soap spray (I usually recommend a very dilute solution of 1 tsp/5 mL liquid SOAP to 1 quart/L of water), perhaps using cotton balls with plain warm water to wipe the sooty mold off will give you a good start on combating this ugly fungus.
Good air circulation will help the plant fend off further attacks, but is not a guarantee so long as aphids/mealybugs/scale can get to it. Ants will "farm" aphids for their sticky secretions ("honeydew"), so ant prevention will also be needed.
But let's look at the presumed sooty mold first.
Sunset Zone 17, USDA Zone 9