Preheat oven to 325°F. Line two 8-inch cake pans with parchment paper and lightly grease with butter.
In a large mixing bowl, combine:
1 box lemon cake mix
1 packet lemon pudding mix
1 cup room-temperature milk
¼ cup melted butter
½ cup sour cream
Mix until smooth.
Add 4 eggs and mix again until fully incorporated and smooth.
Toss ½ cup blueberries in a light coating of flour. Gently fold them into the batter.
Divide the batter evenly between the prepared pans (about 4 cups per pan) and smooth the tops.
Bake for 23 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in the pans. Once cooled enough to handle, remove them from the pans and place on a wire rack to cool completely, about 2 hours.
While the cakes cool, make the blueberry filling. In a saucepan, combine:
2 cups blueberries
¼ cup sugar
1 tablespoon lemon juice
Bring to a gentle boil, then reduce heat and simmer for 5 to 10 minutes, stirring occasionally. Use a spoon to lightly mash the blueberries. Remove from heat and let cool completely.
Place one cake layer on a serving plate. Pipe or spread a ring of frosting around the edge, then fill the center with the blueberry mixture. Top with the second cake layer.
Frost the top and sides of the cake. Garnish with fresh blueberries and lemon slices before serving.
