Make a double pie crust. Place one in a 9" deep pie dish or 10 1/2" regular pie dish.In a large bowl combine the sugar, tapioca and salt. Add rhubarb and blueberries; toss to coat. Let stand for 15 minutes. Transfer to crust. Dot with butter. Place remaining crust over filling. Trim, seal and flute edges. Cut slits in pastry. Brush with milk; sprinkle with sugar if desired. Bake at 400 F. for 40-45 minutes or until crust is golden brown and filling is bubbly. Cook on a wire rack.
Note: If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander but do not press liquid out.