Having had a white walnut tree to content with recently, I have done some research on the walnut trees and their propensity for causing difficulties with plant growth.
The new wisdom is that walnut leaves are okay in a compost pile if they have been left out to dry for a year. After a year, the Juglone has wasted away to negligible levels, and the leaves (or other matter) can be successfully composted. In addition, the root system will stop secreting Juglone approximately one year from the time of the tree's removal.
Again, it bears mentioning that this is the New Wisdom - tradition says never use the walnut leaves (black, or the less common white variety [a.k.a. Butternut]) in compost. Take it for what it's worth. =)