What do you mean "thick layer of black mulch"? What is the black dye used? And what are the purposes of mulching the hard clay?

It sounds like the typical problem of wood chips or bark mulch applied too deeply. When the mulch starts getting to 4 or 5 inches or more deep, sometimes the temps in the mulch heats up and actually dries out the mulch. When fungus spreads throughout the mulch, it can clump up as you described and prevent water from reaching the soil.

It's a case where more is not always better. Adding a finer textured compost under the heavy wood mulch will help feed the microbes much faster, but moisture still needs to reach the soil. (The microbes are fine, they're just parched.) Try keeping only an inch or two of woody mulch on the soil and microbes and worms should start to work on the heavy clay.

Wayne

If the soil under the mulch does not contain sufficient levels of organic matter all kinds of strange things will happen and what you describe is one of them. With some clay soils you do need to add some organic matter into the soil before covering it with mulches or the clay soil under the mulch compacts even more and some mulches will pack down with those mycelium growing through trying hard to digest that mulch.