In a small bowl, combine flour and salt; cut in the shortening until crumbly. Gradually add water, tossing with a fork until dough forms a ball. Divide dough in half so that one portion is slightly larger than the other; wrap each in plastic wrap. Refrigerate for 4 hours or until easy to handle.
On a lightly floured surface, roll out larger portion of dough to fit a 9-in. deep-dish pie plate. Transfer pastry to pie plate. Trim pastry even with edge.
For filling, 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.
Roll out remaining pastry to fit top of pie. Place over filling. Trim, seal and flute edges. Cut slits in pastry. Brush with milk; sprinkle with coarse sugar if desired.
Bake at 400° for 40-45 minutes or until crust is golden brown and filling is bubbly. Cover edges with foil during the last 15 minutes to prevent overbrowning if necessary. Cool on a wire rack. Yield: 8 servings.
Editor's Note: If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.