Combine blueberries, half cup of the sugar and all the lemon juice in a saucepan. Stirring gently, heat mixture until it is simmering and sugar is dissolved. Let cool to lukewarm while preparing cake batter. Cream butter, add quarter cup sugar and cream thoroughly. Add egg and mix well. Sift flour with baking powder and salt. Alternately, add with milk to egg mixture. Add vanilla. Gently, fold blueberry sauce into batter. Pour batter into unbaked pie shell. Bake in a preheated 375 degree oven for 30 to 35 minutesor until cakes tests done.