Preheat your oven to 350ºF (175ºC) and prepare three 8-inch round cake pans by greasing with butter and lining with parchment paper.

In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt.

In another bowl, beat together butter, vegetable oil, and sugar until light and fluffy.

Mix in eggs one at a time and stir in vanilla extract.

Alternate adding buttermilk and dry flour mixture to the creamed mixture, mixing until just combined.

Fold in blueberries gently and divide batter among the prepared pans.

Bake for 30-40 minutes until a toothpick comes out clean.

Cool cakes in pans for 10 minutes then transfer to a wire rack to cool completely.

For the blueberry jam, cook blueberries, sugar, and lemon juice until thickened, stirring continuously.

Blend blueberries for puree and cook until slightly thickened.

Beat cream cheese and butter until creamy, add powdered sugar and blueberry puree until fluffy.

Assemble cakes with layers of blueberry jam and frosting, then frost the entire cake.

Chill and decorate with fresh blueberries and mint leaves before serving.