Kent,
United Kingdom 
black sooty deposits on leaves of mature camellia what is causing it?
6 Jun, 2010
Previous question
« Thank you Fractal, I wasn't sure if I needed a cloche or something. :-)
The sooty deposit will be secondary. An insect such as scale or some sort of aphid will be sucking the sap. This drops onto the foliage below and the sooty mould lives off the sticky sugary residue.
Check under leaves for scale insect or aphids on new growth and treat accordingly. The sooty mould can be gently washed off or wiped away after soaking with water and maybe a little washing up liquid added.
6 Jun, 2010