Posted: 12/05/2012 at 15:37
They're scale insect. They suck sap and excrete sugar, and this drips onto the top of the leaves and mould grows on it, hence the black leaves.
Spray with Provado Ultimate Bug Killer, making sure that you wet the underside of the leaves, and that will kill the scale insect. You can then wash the leaves after a couple of weeks to get rid of the mould.
If the insects appear again, spray again until they're gone.