Preheat the oven to 375°F (190°C).
In a medium mixing bowl, combine the blueberries with 1/4 cup of the granulated sugar and the lemon juice; toss gently to coat and set aside.
In a separate large bowl, whisk together the flour, the remaining 3/4 cup of sugar, baking powder, and salt.
Gradually whisk the milk, melted butter, and vanilla extract into the flour mixture until the batter is smooth and well combined.
Pour the batter into an ungreased 9x9 inch square baking dish or a 2-quart casserole dish.
Spoon the blueberry mixture and any accumulated juices evenly over the top of the batter. Do not stir.
Bake for 40 to 45 minutes, or until the topping has risen around the fruit and is golden brown and a toothpick inserted into the cake portion comes out clean.
Remove from the oven and allow to cool for at least 15 minutes to allow the fruit filling to set before serving.