Sycamore tar spot.

These black spots on the leaves of a sycamore tree  are a fungus known as the sycamore tarspot Rhytisma acerinum. The don't appear to harm the tree although often  present in huge numbers. They are very sensitive to sulphur dioxide and so the more you have in your area the purer is the the air.

For some reason they always bring to my mind  Robert Louis Stevenson's novel Treasure Island and the dreaded black spot.