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, with 1 portion slightly larger than the other. Wrap separately; 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. cast-iron skillet or deep-dish pie plate. Transfer crust to pie plate. Trim crust even with rim of plate.
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 dough to fit top of pie. Place over filling. Trim, seal and flute edge. Cut slits in crust. If desired, make additional dough and use to create cutouts to decorate top of pie. Brush with milk; sprinkle with coarse sugar if desired.
Bake at 400° until crust is golden brown and filling is bubbly, 40-45 minutes. Cover edge with foil during the last 15 minutes to prevent overbrowning if necessary. Cool on a wire rack.