My son lives in central Ohio and has black walnut tress in his back yard. When we visit him during the fall I can't resist picking the walnuts off of the ground because they appear to be 1/3 bigger than those I harvest in Wisconsin. I pick some that have green husk and some that have dark brown husk. I remove the husk, spread them in a single layer and turn them as they dry. I have done this three different years and the result is always the same. When I crack them the meat is dark black and shriveled. What is causing this? Are there different varieties of black walnuts?
There could be several things going wrong with the black walnuts. One of the more common causes of the shriveled nut meats could be a fungus disease called anthracnose. This disease mostly affects the leaves of walnut and many other tree species, especially in years with wet springs. However, it can also cause the nutmeats to become dark and shriveled.
These articles discuss this disease in a bit more detail:
There's nothing that a home gardener can do about this, because of the difficulty in getting pesticide spray up into the canopy of a large tree.
Your son can bring some of the nuts (and leaves, next year) to his local county extension office to have them examined, and possibly sent to Ohio State's plant diagnostic lab.