Ficus Bonsai Leaves Turning Black

Let us go over the main factors that can cause the tree to fade.
Ficus bonsai leaves turning black. Often a sure sign of a pest problem will be that the leaves on the ficus tree will be sticky or have liquid dripping off them as well as falling off. Ficus leaves may also turn yellow if the humidity is low. This disease occurs more frequently in warm humid conditions like leaf spot but often follows tissue damage from other causes. This fungus mostly attacks foliage and will appear as black spots or patches on the leaves.
Remove the diseased leaves and spray with benlate follow the instructions exactly. Once a leaf is infected it must be removed from the tree to prevent spreading. If this is the problem you will need to treat the plant with insecticide like neem oil. In that case fill the water tub with distilled lukewarm water and place the pot in it.
Fix the problem by watering your ficus tree every time its soil dries out to a depth of an inch. A fungal disease anthracnose is characterized by necrotic spots on the leaves and commonly causes tip dieback in ficus plants grown indoors. From your picture i can understand the black leaves are due to over moisture. Pests ficus trees are susceptible to a few pests that can cause a ficus tree to drop leaves.
Yellowing leaves often occurs on indoor ficus during the winter when indoor air tends to be dry. Most likely the plant is feeling thirsty and the soil mix in the pot is completely dry. This looks like tiny black dots on the backs of the leaves of the tree. If your ficus plant is under watered its leaves will turn bright yellow all over the plant and then drop off.
The leaves may turn yellow and fall off. Ficus trees require regular watering. The spots initially appear as dark fuzzy mold spots on the top side of the leaves. Yellowing of your ficus leaves is another clear indicator of water stress.
Black spots on the top of the ficus leaves is the first sign that a fungal disease is infecting the tree. The plants kept indoors will show us some indication of over watering and insufficient sunlight by turning the leaves to yellow colour. These are also known as cercospora spp. Eventually the leaves will yellow shrivel and drop off.
Spray healthy foliage with a fungicide. If the soil feels moist mist the leaves to increase humidity around the plant. If you let your plant dry out it may respond by shedding its leaves.