Sooty moulds are a group of fungal growths which colonize sugary deposits on plants. These deposits may include sap dripping from pruning wounds or frost cracks or honeydew from feeding insects like aphids, scale or mealybugs.
Even though maples have light coloured bark, annual aphid feeding provides a reliable source of honeydew for sooty mould colonization, which often turns the bark black.
This is an aesthetic problem only, and poses no risk to the tree.
Sooty moulds can be indirectly controlled by reducing populations of sucking insects that excrete honeydew. Outdoor furniture can be hosed down with water during periods of honeydew excretion, particularly during drought.
The growth of sooty mould fungi is inhibited by preservatives used in treated wood in rustic outdoor furniture. Recipes for cleaning solutions to remove sooty moulds from plastic or painted surfaces can be found on the internet.