Oh, I "see" better now. That looks like a bad infestation by insects! And they may already be gone because they tend to visit when leaf out starts. Lacebugs, aphids and other insects excrete honeydew, usually on the leaves' bottom surface, which attracts ants and promotes the development of a black sooty mold everywhere in leaves and branches. Once they are done, the insects vanish but leave the black sooty mold throughout all parts of the plant. The insects could also be located on another plant, if said plant is right above the azalea. The sooty mold -if that is what the blackish color is- does not feed on the plant and naturally falls down.... but at its own , very slow pace.
I had a light case of lacebugs a long time ago. The tops of the leaves had white dots, where the lacebugs sucked juice from and the bottom had black sooty mold spots. You have a much worse case of whatever insect is behind this. I caught my problem in time and used a systemic insecticide for 2-3 years in Spring.
You may able to combat the insects now (if they are still there) or in future years by releasing beneficial insects like ladybeetles or parasitic wasps at leaf out time. The ladybeetles are sold in many plant nurseries but the wasps tend to be in stock only at organic minded stores. A hard stream of water from your hose will also kill them because they tend to be snacking on the leaves at most times and the hard force of the water will literally break the rest of them off the leaves. Some of the insects, of course, will not be injured and will fall down (but they will climb back again) so, it pays to throw some water at the plant for several days in a row at the bush.