Greetings Sherry:
New seedlings come from the the fruit (e.g., nut) of black walnut trees. In addition, vegetative sprouts may originate from the basal area of an existing walnut tree (i.e., basal sprouts or suckers).
Based on my experiences with direct seeding black walnut here in the Midwest, I've noticed that black walnut seed may lay dormant in the soil for up to two years.