In a greased 5-qt. slow cooker, combine blueberries, sugar and cornstarch; stir in vanilla. In a large bowl, combine cake mix and melted butter. Crumble over blueberries. Top with pecans. Cover slow cooker insert with a double layer of white paper towels; place lid securely over towels. Cook, covered, on low until topping is set, 3-4 hours. If desired, serve with ice cream.