To shell black walnuts, first use the proper tool. Hand-held tools are rarely adequate for black walnuts because of the hardness of the shell; use a hammer instead. You can also precondition the shells by soaking them before cracking. Black walnuts contain a dye, so wear gloves and old clothes.
Preconditioning entails soaking the nuts in hot water for 1 to 2 hours, draining, and keeping the nuts moist overnight in an airtight container. This should soften the shells. If they still seem hard, soak them in hot water for another hour. Once they are ready to be cracked, place the nut on a cloth or tarp with the point of the nut facing up. Have your tarp spread on a hard surface, such as concrete. Strike with the hammer until it splits in half, then hammer it squarely until it splits open. Pull remaining shell apart and pry the nutmeat loose.