Ammoun, I am not sure what that is. Maybe your guess is right, if the black spots do not rub off.
Bovine, Black Sooty Mold, and under the leaves you will probably find mango scale. Spray undersides of leaves with an insecticide.
Eddi, follow directions on labels for amount per gallon, time of day and temperature during spraying time. Oils only work during slow-drying conditions.