The black walnut is a delicacy worth working for. The smoky-sweet nut is seldom available from retailers, but the black walnut tree, Juglans nigra, is commonly grown as an ornamental. If you're harvesting your own nuts, you'll put a lot of effort into preparing them for proper storage. It would be a shame to go to all the trouble of getting black walnuts almost ready, just to open the treasure chest and find a rotten nutmeat inside. It's a good thing there are a couple of little things you can do to tell whether a black walnut is good or bad.
Put on rubber gloves and wear old clothes and shoes that you're not worried about ruining. The hulls of black walnuts contain a strong dye-like substance that will stain your skin, as well as any hard or porous surface.
Look the black walnut over. The hull should be bright yellow-green, like a tennis ball. There will undoubtedly be brown or black spots or blemishes on it here and there, but that's all right. If the black walnut has a lot of black or brown on it, or it's mushy, then it's rotten and the nut inside may be rotten, too.
Set an unhulled black walnut on a solid surface that you're not afraid to ruin, such as an old board or concrete block. Stomp on it hard, and roll it around under your foot. The thick, green hull will crack open, revealing the nut inside.
Look at the inside of the hull. If it's brown or black it's rotten, and the nut probably won't be any good. Discard the hull.
Drop the hulled black walnut in a bucket of water. If it floats, throw it away because it's rotten. If it sinks, it's a keeper.