Camelia

The new growth on my camelia bushes have turned black. They have not done this before, any ideas?

Are they in pots which have got too dry at some stage?

No, they are planted in the ground and have been for the last three years. The soil is slightly acidic and the spot is in the shade for the morning, but in sun in the afternoon. The shrubs flowered great this year, we have had continuous rain for about three weeks and temperatures are below average for the time of year.

Have the leaves turned black or is it residue that will rub off?

The black is it not residue, they have tuned black - as if burnt!

Frost?