It looks like sooty mold, a fungal growth. Several species of fungus happens to grow on the sugary substance (honeydew) secreted by sucking insects like aphids, whiteflies, hoppers, scale insects etc.,
Generally, sooty mold won't directly affects plants but might reduce photosynthesis by blocking the sunlight.
It can be easily removed by using the strong jet of water or spraying liquid starch to flake out the fungus.
Sooty mold growing on honeydew produced by citricola scale, Coccus pseudomagnoliarum, on orange.
Some common honeydew-producing insect groups.
Citricola scale-infested leaves showing sooty mold growth.
Ash whitefly, Siphoninus phillyreae, honeydew on leaf.