Preheat the oven to 350F; butter and flour a 9-inch round cake pan.
For the crumble topping, use a fork to combine the sugar, flour, salt, and vanilla, and then cut in the butter until it looks like coarse meal. Set aside for now.
For the batter, sift together the flour, baking powder, baking soda, and salt in a medium bowl and set aside.
In a large bowl, cream together the butter, oil, and sugar, and then beat in the eggs, buttermilk, and vanilla.
Stir the dry ingredients into the wet half at a time, being careful not to over-mix. Gently toss together the blueberries and 1/2 tablespoon flour. Fold the blueberries into the batter.
Pour the batter into the prepared cake pan and sprinkle on the crumble topping.
Bake until a wooden pick inserted in the center comes out clean, about 1 hour, to 1 hour, 15 minutes.
Cool completely before cutting and serving.