I thought they looked more like Cottony cushion scale. Scales are common pests of citrus trees. The black stuff is probably sooty mold that grows on the sap that the scales are sucking out of the tree. The mold grows on the sap.
The mold itself will go away once the scales are taken care of.
Treat the area for ants by putting out ant baits. Terro works on sweet eating ants.
Here is another ant bait recipe: Works for sweet and grease eating ants. measuring spoons, cups and containers should be dedicated for this task just in case. I even have a jar of goober labeled just to make bait.
Mix three parts peanut butter with two parts jelly and add one tablespoon of boric acid per six ounces of mix. Place the bait on pieces of paper so stuff it into large straws and place where you see the ants foraging. Again keep out of the reach of pets and children.
Ants will farm aphids and scale and protect them from their predators.
Scale adults are hard to kill, horticultural oils work better than soap. I use a soft brush and a smaller toothbrush and alcohol to scrub them off the branches and leaves. Then hose off with water. Even if scale is killed by the oil, the shell will remain on the branch. Once the scales are dead, there will be no sap for the mold to live on and it can be washed off with water.