Preheat your oven to 350ºF (175ºC). Grease three 8-inch round cake pans with unsalted butter or vegetable oil and line the bottoms with parchment paper.

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

In another large bowl, beat together the softened butter, vegetable oil, and granulated sugar until light and fluffy, about 4-5 minutes.

Crack in the eggs one at a time, mixing thoroughly after each addition. Pour in the vanilla extract and mix until smooth.

Gradually add buttermilk and the dry mixture to the butter mixture, alternating between the two, mixing gently until fully incorporated.

Divide the batter and fold blueberries into one section gently. Pour undiluted batter into each pan and distribute blueberry-infused batter evenly among the three pans.

Bake in the preheated oven for about 30-35 minutes until golden brown and a toothpick comes out clean.

Let the cakes cool in the pans for 10 minutes, then transfer to a cooling rack to cool completely.

Prepare blueberry jam by cooking fresh blueberries with sugar and lemon juice until bubbling. Thicken with cornstarch slurry if needed.

In a bowl, beat together softened butter and cream cheese until fluffy. Gradually add powdered sugar and blueberry puree, mixing until smooth.

Assemble the cake by spreading blueberry jam and frosting between the layers, crumb coating the entire cake before refrigerating for 30 minutes.

Apply a final layer of frosting and decorate with fresh blueberries and powdered sugar before serving.