Mix blueberries, 1/2 cup of the sugar and lemon juice in a medium saucepan. Cook over medium heat stirring frequently until sugar is dissolved and mixture simmers. Remove from heat and set aside to cool to lukewarm. Preheat oven to 375 degrees. While blueberry mixture cools, cream butter with remaining 1/2 cup sugar and vanilla until completely blended. Stir in egg and beat until light and fluffy. Thoroughly combine flour, baking powder and salt in a medium bowl. Add half of the flour mixture and half of the buttermilk to the butter and sugar mixture, beating gently until smooth. Add remaining flour and buttermilk mixtures and beat till smooth. Fold in the blueberry mixture with a large spoon and mix just enough to create a marbled effect. Place pie shell on a baking sheet. Pour batter into unbaked pie shell. It will be very full. Bake for 30 to 35 minutes or until a wooden pick inserted in the center comes out clean.