You can use it. It's similar to other nutwoods but a bit stronger. Some parts of the of black walnut produce toxins but this is not an issue when smoking. It's not good around horses, btw.
I've used it a couple of times. I didn't die or anything. It had a dark, intense flavor that I liked in the small quantity I used it in. I bet it could get overwhelming, but I'm not afraid of using it in small quantities.