It could be the "dreaded" sooty mould which is usually more an aesthetic problem than health. This comes about through infestations of honeydew producing insects such as Aphids.It is the secretions that bring about the sooty mould.However if the tree is heavily coated with mould this may prevent photosynthesis. Cut back discreetly on a couple of branchlets, or twigs and if the core shows green you have life!! Then you need to clean of the mould which should be quite easy with a hose.