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Never used it, but the tree does produce a nut so it should be ok to use. I would test it sparingly. Black Walnut has a lot of tannin and it might make a slightly bitter smoke.
I had a big old black walnut tree in my back yard at the first house we ever owned. In the spring it was just like mowing a gatling gun, them black walnuts were firing everywhere!
I've used it numerous times and never noticed a bitter taste. It is definitely milder than hickory and mesquite.
One of the main reasons I made my mini-WSM was to try different woods. I can smoke a couple chickens and some sausage in it and see what flavor different woods impart. The only conclusion I've come to is that everything that comes out of it tastes great!