Certified GKH Gardening Expert
It sounds like it may have walnut blister mite. This article will help:
The black fungus may be attracted to the weakened tree. There are a few kinds it could be, but they can all be treated with fungicide.
To tell the truth, I would recommend treating the tree with neem oil. It is both a fungicide and a miticide and is safe for people and pets. Here is more information on it: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm