The walnut husks almost never turn black on the tree. They fall off green and then turn black. The walnut must cure and dry out some before it can be easily ('easily' being a relative term here) shelled. One way is to put on old boots and stomp the walnuts to loosen them from the husks. Exposed to air they will turn black in a few hours. Put them somewhere they can dry for several days to several weeks before trying to shell them. Until they are fairly dry, you cannot allow them to freeze.
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