In theory, black walnut is one of the softer hardwoods with ash and maple being 30-40% "harder" so to speak. As CC said, walnut is a popular choice with stock builders in respects to the woods work-ability, beauty, availability, and weight (it's less dense than some of the other choices). With a stock, you want to make sure that your blank has uniform grain for strength (i.e. free of knots) and the hardware is fitted correctly without binding against the stock anywhere as it will help alleviate any proneness to crack.