I have seen a couple of warnings about potential harm from extended consumption of black walnut.

http://wellnessmama.com/257/black-walnut-hull-herb-profile/

https://www.mountainroseherbs.com/products/walnut-leaf/profile

http://www.starwest-botanicals.com/category/black-walnut-leaf-extract-organic/?utm_source=froogle&utm_medium=feed&gclid=CMqChrew4MACFSgV7AodOBIAxQ