Black walnuts are hidden inside these things that look like green tennis balls that fall from trees. They stink, they stain everything, and it's confoundingly difficult to get the green fibrous crap off the outside. This is one of those deals where the harder you try, the less progress you make. The best way is laissez faire: dump your harvest of tennis balls in the ruts of your driveway and drive over them until you see the brown nut shells inside.
Note: squirrels are very well equipped to deal with black walnuts. Don't leave them outside indefinitely.
Once you've got the brown guys pictured above, put them into a couple layers of tough plastic bags and beat on them with a hammer. It should then be possible to get most of the nutmeats out. Like I said, this is labor intensive.
Here's a recipe that raises eyebrows:
8 oz semisweet chocolateCombine the ingredients in the usual cookie way. Bake at 325 for 12-15 minutes.
3 oz unsweetened chocolate
8 tbsp unsalted butter
1 1/4 c sugar
2 tsp vanilla extract
2/3 c bleached all-purpose flour
1/2 tsp taking powder
1/4 tsp salt
1 1/2 c (about 9 oz) chocolate chips
1 1/2 c (about 6 oz) chopped black walnuts
1 1/2 c (about 6 oz) chopped pecans