You probably already know this, but for the sake of anyone reading who isn’t aware: never use black walnut anything in the garden. All parts of the tree leach a chemical that inhibits the growth of other plants. That extends to the leaves as well.
On the other hand, I might use black walnut leaves to mulch a path.
A very useful tip. I won't even plant a black walnut on the property 'cause one of the oldtimers around here told me I shouldn't. Not sure how far away it would have to be.