Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the flour, sugar, baking powder, and salt.
In another bowl, mix together the milk, melted butter, vanilla extract, and lemon juice.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the blueberries.
Pour the batter into a greased 8-inch square baking dish.
In a separate bowl, mix the additional 1/2 cup of sugar with boiling water until dissolved.
Slowly pour the hot sugar water over the batter in the baking dish. Do not stir.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown.
Allow to cool for a few minutes before serving.