Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and set aside.
In a small bowl, mix the melted butter and brown sugar until well combined. Spread this mixture evenly onto the bottom of the prepared cake pan.
Arrange the fresh blueberries on top of the butter-sugar mixture in a single layer.
In another bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large mixing bowl, cream the softened butter and granulated sugar until light and fluffy.
Beat in the eggs, one at a time, followed by the lemon zest and lemon juice.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, and mix until just combined. Be careful not to overmix.
Pour the batter over the blueberries in the cake pan, spreading it evenly.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before inverting it onto a serving plate.
Serve slices of the lemon blueberry upside down cake warm or at room temperature. Enjoy!