Ruth Lawrence's Blackberry Pie

Ruth Lawrence's Blackberry Pie
Source: From Four-and-Twenty Blackbirds to Basic Apple, It's as Easy as Pie by Susan G. Purdy (I've checked out several books by her and she seems quite good.)
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Cooking the berries ahead of time is a bit of trouble but worth it!

She says that the vinegar is supposed to soften the seeds. I've heard that this is folklore, but...who knows? She also says that these pies can be frozen unbaked and then taken out of the freezer and baked while still frozen and taste great:
--don't cut air vents in top crust
--wrap pie airtight in freezer-weight material (I like to stick things in freezer bags when I can b/c they're reusable--a great tip I got from my old aunts is to double bag everything in the freezer)
--don't forget to label!
--to bake:
--set pie in lower third of a 450F preheated oven for 20 min
--after the first 10 min, cut steam vents in the top crust
--after 20 min, lower heat to 350F, raise pie to oven center and cover pastry edges w/ foil or protector if browning too fast
--continue baking an additional 40-45 min or until pastry is gold brown
--if you want to glaze pie top, apply egg glaze and a sprinkling of sugar before setting pie in oven
--cool on a wire rack

I've never done that but if you've got a bush, you might get a lot of berries!
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Baking time: 425F for 10 min; 350F for 40-45 min
Berry-cooking time: 5 to 7 min
Makes one double-crust 9-inch pie

Unbaked pastry for a 2-crust, 9-inch pie
Egg glaze: 1 egg beaten with 1 T water
4 cups fresh blackberries, picked over and quickly rinsed
1 1/2 to 1 2/3 cp granulated sugar (depending on the sweetness of the berries and your taste)
1/2 tsp cider vinegar
3 T cornstarch
1 T quick-cooking tapioca

1. Prepare the pastry, roll it out, and line the pie plate. Trim a 1/2-inch pastry overhang. To moisture-proof the lower crust, brush with egg glaze. Chill the crust in the fridge while you prepare the filling. Preheat oven to 425F.

2. In a heavy-bottom saucepan, combine berries, sugar, vinegar and cornstarch, and add 2 T water (to prevent scorching). Mash the fruit very slightly with a potato ricer or spoon in order to start the juices flowing. Set the pan over medium-low heat and cook, stirring occasionally, just until the mixture nears the boiling point. (You don't want to overcook cornstarch.) Remove from heat and cool completely. Stir in tapioca.

3. Add the cooled fruit to pastry-lined pan. Brush egg glaze over the edge of the lower crust. Roll out top crust and fit over pie. Trim a 3/4-inch overhang. Fold the top edge under the bottom crust overhang and pinch them together to seal, making a raised rim all around. Flute the edge. Cut vent holes in top crust.

4. Set pie in lower third of preheated oven and bake 10 min. Reduce heat to 350F, raise pie to center of oven and bake an additional 40-45 min, or until pastry is golden brown. Check pie after about half the total baking time and add a foil edging or protector to keep crust edge from overbrowning. Cool on a wire rack. Serve warm or cold.