1. Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. In a medium bowl, whisk together the flour, baking powder, and salt.
3. In a large bowl, cream together the butter, oil, and sugar until light and fluffy. Beat in the vanilla, then add the eggs one at a time, beating well after each addition.
4. Gradually add the dry ingredients to the wet ingredients, alternating with the milk and beginning and ending with the dry ingredients. Mix until just combined with paddle attachment..
5. Fold in the sour cream, lemon zest, and lemon juice. Gently fold in the blueberries.
6. Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
7. Let the cakes cool completely in the pans before frosting.

Making the Frosting:

1. In a large bowl, cream together the butter and powdered sugar until light and fluffy.
2. Beat in the salt and lemon extract.
3. Add the heavy cream and beat until smooth and spreadable.

Assembling the Cake:

1. Place one cake layer on a serving plate or cake stand. Spread a layer of frosting on top.
2. Top with the second cake layer and frost the top and sides of the cake.
3. Decorate with additional blueberries, lemon zest, or lemon slices, if desired.