Place blueberries in a deep 8-inch pie pan; drizzle lemon juice over blueberries.
Beat 3/4 cup sugar and butter together in a bowl using an electric mixer until creamy. Sift flour, baking powder, and 1/4 teaspoon salt together in a bowl. Mix milk, alternating with flour mixture, into creamed butter mixture until batter is just combined. Pour batter over blueberries.
Combine 1/2 cup sugar, cornstarch, and 1/2 teaspoon salt together in a bowl until thoroughly mixed; sprinkle over batter. Pour boiling water over sugar mixture.
Bake in the preheated oven until a toothpick inserted in the middle of the cake comes out clean, 45 to 50 minutes.