Lived in the land of the black walnut (the Ozarks) a long time, just never heard of anyone using it to smoke with. Folks cut a black walnut tree to sell the log for walnut lumber or veneer, a veneer log brings more money. The large limbs cut from the log are generally burned for heat in wood stoves.
Otherwise, the walnuts are picked up off the ground in the fall, hulled and sold to Hammon's Walnut Products in Stockton, MO.
It is a hardwood though, suppose you could smoke with it.