The interesting thing about black walnuts (Juglans nigra, Juglandaceae) and walnuts in general is that they’re not true botanical nuts. They’re actually drupes, much like peaches or plums. The walnut itself if surrounded by a fleshy husk, similar to a peach pit within a peach.
More specifically, a walnut is actually called a tryma, which is a nut-like drupe. The “nuts” of hickory trees (also in the family Juglandaceae) are also trymata.