I have a black fungus on all my trees of every variety from maples to tulip...
I have a black fungus on all my trees of every variety from maples to tulip poplar. It is in patches and is growing larger. The trees are quite old. I have been on this property for 46 years and some of the trees have been here since then. I contacted the extension service last year about the problem, but they did not seem concerned. I think all the trees are dying. My next door neighbor has the same problem. I am not sure about other neighbors.
From your description, it appears what your describing is sooty mold, a condition caused by "honey dew" deposited from aphids or scale insects. See the attached pub. http://ohioline.osu.edu/hyg-fact/3000/3046.html.
The black fungal growth can appear on stems and twigs as well as the leaves. The attached publication also list trees and shrubs susceptible to this condition.