Wash it off with warm water, with a few mild soap flakes dissolved into it - don't use detergent.
If it's an ongoing problem, you might need to improve the ventilation around the plant (judicious pruning to remove overcrowded stems) and check for any bugs which are leaving honeydew around (greenfly etc)

The current relatively warm damp weather won't be helping.

You can find a pruning instructions at http://www.trehane.co.uk/kb_results.asp?ID=7

I agree with urbanite, solve the problem of what is leaving the honey dew for the sooty mould to feed on. then wash off the sooty mould.

It's most likely cotton scale causing the problem - sooty mould appears in relation with it, and Camellias are prone to this form of scale. The best time to treat the scale is around late June/early July, when the nymphs hatch out, when you should spray thoroughly, including the underside of the leaves, with something like Provado Ultimate Bug Killer Concentrate or Bug Clear Ultra. Get rid of the scale and the sooty mould goes, but in the meantime, you can wash it off as described by Urbanite.