1. Preheat the oven to 350°F (180°C). Grease and line three 8-inch round cake pans with parchment paper, then grease the parchment as well.
2. In a large bowl, beat the butter until smooth, then add the granulated sugar. Continue beating until creamy. Add the oil and beat until the mixture is light and fluffy. Incorporate the eggs one at a time, then add the vanilla until well combined.
3. In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
4. Gradually add the dry ingredients to the wet mixture, mixing briefly until just combined.
5. Stir in the buttermilk, lemon zest, and lemon juice until the batter is smooth.
6. Toss the blueberries in a tablespoon of flour to prevent them from sinking, then gently fold them into the batter.
7. Evenly distribute the batter among the prepared cake pans.
8. Bake the cakes for 22–26 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool completely in their pans.
9. For the frosting, beat the softened cream cheese and butter together until smooth. Add the confectioners’ sugar, heavy cream, vanilla, and a pinch of salt, then beat on high speed until fluffy.
10. Trim the tops of the cooled cake layers to level them if necessary.
11. Spread frosting between each cake layer, and then cover the top and sides with the remaining frosting.
12. Decorate with extra blueberries or lemon zest, if desired.
13. Chill the assembled cake for at least 45 minutes before slicing and serving.