nurtomiBlack sooty coating
A dark surface coating may grow on sticky honeydew.
What to look for
Check whether the black film can be gently wiped and whether leaves are sticky. Inspect stems and leaf undersides for aphids, mealybugs or scale that may be producing honeydew.
What to do
Address any insects you actually find, and gently clean suitable leaves with water and a soft cloth. Check nearby foliage again; removing the coating alone will not solve an ongoing insect problem.