Greetings Michael:
Because standing timber, logs, and 1-percent of yard trees must be evaluated in-person to ascertain quality (defects), highest-use, and potential value, we simply cannot "value" timber, logs, or trees via this forum -- it's just not possible and I hope you can understand those limitations.
Therefore, your best bet is to contact your local state forestry/Extension agency for follow-up expertise regarding the "potential" marketing of your black walnut yard tree:
CA Extension Forestry: https://ucanr.edu/sites/forestry/
Best of luck!