I have about a dozen or so Black Walnut trees and never had a problem with my birds. The only thing I know about the trees is that it can keep nightshade plants from growing properly (tomatoes, potatoes, peppers) and can cause laminitis in horses. My birds pretty much leave the leaves alone.
But as a bedding I would make sure that they are and stay dry. Wet leaves can cause mold to grow and the mold can be a major health problem for the birds. Mixed in with other litter that stays dry should not be a problem. Just remove it if/when it gets wet.