Preheat the oven to 350 degrees F (175 degrees C).
Mix all-purpose flour, cornmeal, baking powder, and salt together in a bowl.
Cream together butter and sugar with an electric mixer until smooth. Beat in eggs one at a time, scraping down the bowl after each addition. Add sour cream and vanilla; combine until smooth. Add flour mixture and mix until incorporated. Set aside.
Divide butter between two 9-inch cast iron pans; melt over medium-low heat, about 1 minute. Add 1/2 of the brown sugar to each pan; cook until butter and sugar begin to bubble, 2 to 3 minutes. Divide blueberries between the two pans and remove from stove top.
Divide cornmeal batter between the pans; place each on a sheet pan.
Bake in the preheated oven until a toothpick inserted into the middle comes out clean, 45 to 50 minutes.
Let cool slightly, about 15 minutes. Run a knife around the outer edges of each cake and invert onto a cutting board for slicing.
