To make the crust, combine the flours and salt in a bowl. Use your fingers or a pastry cutter to cut in half of the butter until it is the size of peas, then cut in the other half until it is the size lima beans. Some of the butter will be completely worked into the flour, but you should have lots of visible pieces of butter in the dough, too.
Combine the water and apple cider vinegar in a measuring cup. Make a well in the flour/butter mixture and slowly stream about 6 tablespoons of the water-vinegar into the dough while mixing gently. If the dough seems very dry, add more water a couple of teaspoons at a time. You have added enough water when you can pick up a handful of the dough and squeeze it together easily without it falling apart. Press the dough together, then split it in half, form into disks, and wrap each disk in plastic wrap. Chill the dough for at least one hour before using, or overnight (preferred).
When you're ready to make the filling and assemble the pie, preheat the oven to 400º F. In a large bowl, combine the sugar, lemon zest, and vanilla bean seeds. Use your fingers to rub the zest and seeds into the sugar, until well combined and fragrant. Stir in the flour, cinnamon, and salt.
Cut the rhubarb into 1-inch pieces and add it to the bowl. Add the blackberries and lemon juice, but don't stir quite yet.
On a lightly floured surface, roll out one piece of the dough into a roughly 12-inch circle about 1/8-inch thick and place it into a 9-inch pie pan. Place in the fridge while you prepare the rest of the pie. Roll out the other piece of dough into a roughly 12-inch circle, about 1/8-inch thick, and place it on a parchment-lined baking sheet. Chill that too.
Stir the filling ingredients together, then fill the pie shell and top with the second crust.
Trim the edges so they are even, then crimp them together. Alternately, cut the second crust into 1 1/2- to 2-inch strips and weave a lattice top.
Slide the whole pie into the fridge or freezer for about 15 minutes, or until the crust is very firm. When you are ready to bake, carefully and gently brush the top of the pie with a beaten egg and sprinkle with a healthy dose of coarse sugar.
Put the pie on a baking sheet to catch any drips and bake for 45 to 55 minutes, or until the crust is a deep golden brown and the juices bubble. Cool the pie before slicing and serving.
Yossy Arefi is a photographer and stylist with a passion for food. During her stint working in restaurant kitchens, Yossy started the blog Apt. 2B Baking Co. where, with her trusty Pentax film camera, she photographs and writes about seasonal desserts and preserves. She currently lives in Brooklyn but will always love her native city of Seattle. Follow her work at apt2bbakingco.blogspot.com & yossyarefi.com.