Web Master:
Black mould on leaves tends to be a sign of insects that product honey dew, these tend to sap suckers like aphids, mealy bugs, whiteflies and soft wax scales. Before you hose the leaves to remove the sooty mould look for insects living on the leaves that will give you an idea of the cause. If you are unsure of what you have found it is best to take a branch to your local nursery and ask them for advice.
If the tree is large it may not be possible to treat the tree with chemicals; if the tree is small you could spray with a systemic pesticide or place Bug Killa around the tree's drip-line, all chemicals will affect all insects on the tree included beneficial ones. You may find that regular watering of the leaves will remove the insects as well as the mould.