It looks like "spalted" yew, to me. Early wood rot. Spalted wood can be desirable depending on what you use it for. I don't see why it wouldn't hurt to divide the yew into billets as you normally would. Yew'll see how far it goes for certain. If it goes all the way through, I wouldn't think it would make a worthy selfbow but might make some nice laminates, duck calls, pen blanks or other non-structural projects.