First, your picture needed improvement!
Your 'blight' is sooty mold growing on the sugary excretions of certain insects. Possibly scale insects or whitefly. These mainly live on the undersides of nearby leaves or on stems. The mold itself is not the problem, you have to find and deal with the insects which are not actually visible in your photo.

Many thanks, I've found minute whiteflies under the leaves and I've also found a few leaves with leafminers. I have read advice from other sites which I shall follow.