Add eggs and buttermilk slowly into the food processor so the finished pie does not split.
In a food processor, pulse together flours, sugar and salt until combined. Cut butter into cubes and pulse into mixture until is makes a coarse meal, with visible small chunks. Add in the buttermilk and pulse just until moistened. It should still have small pieces and not yet formed into a ball.
Empty mixture into a lightly floured flat surface. Using your hands, press mixture together until it forms a ball. Wrap in plastic wrap and chill at least 45 minutes or overnight.
When ready to prepare pie, place dough on floured surface. Use a rolling pin to create an 11-inch circle, sprinkling both sides with flour as needed. Roll dough around pin then unroll into a 9-inch pie or tart pan. Press the dough into the bottom and sides of dish and trim off any excess. Store in fridge until ready to add filling.
Preheat oven to 350 F. Place sugar, butter and flour in food processor, pulse until combined. With machine running slowly, pour eggs in a thin stream just until combined. Scrape sides, the start machine again and slowly pour in buttermilk in a thin stream just until combined (this is critical to create an emulsion). Quickly pulse in orange zest and vanilla.
Evenly place blueberries in bottom of pie shell. Pour buttermilk mixture over berries. Place in center of rimmed baking sheet and bake about 40 minutes, or until edges are golden and center is set. Cool completely on a wire rack before serving.