In a large bowl, mix together flour, sugar, and salt. Add cubes of butter and use your hands to work butter into flour, until a shaggy dough starts to form. Combine water and vinegar and add to dough. Knead dough until it comes together.
Divide dough into two even pieces. Shape each into a disc and wrap in plastic. Chill for an hour while you make the filling.
In a large bowl, mix together blackberries, sugar, cornstarch, salt, vinegar, and sage. Use a wooden spoon to gently mash blackberries until they break down and release juices. Set aside until dough is ready to go.
Preheat oven to 350F.
On a lightly floured surface, roll out one of the discs of dough to about 13 inches in diameter. Transfer to a 9 inch pie dish, letting the sides hang over by about 1 - 1½ inches. Roll the second pie disc of dough to about the same size.
Pour blackberry filling into the pie dish and spread evenly. Place the top crust on top of the filling and work your way around the pie, pressing the top and bottom crusts together and crimping, if desired. Brush crust with egg wash and cut a few slits in the top to allow steam to escape.
Place pie on a baking sheet (this pie is juicy and may bubble over) and bake for 90 minutes. Let cool for at least 4 hours before serving.