In a small bowl, combine the blueberries, sugar and walnuts; let stand for 15 minutes., In a large container, layer a third of the frozen yogurt and half of the blueberry mixture. Repeat layers. Top with remaining frozen yogurt. Swirl mixture; freeze until firm., In a small saucepan, combine the chocolate, milk and vanilla. Cook and stir over low heat until chocolate is melted. Serve with frozen yogurt.