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 ½ 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 and 15 minutes.
Cool completely before cutting and serving.
Notes
To Freeze: Wrap the cake well with aluminum foil. Place it into a zip-top freezer bag. Label the bag with the date and the contents. Freeze for up to 3 months. Let the cake come to room temperature before serving.
Here are a couple of my favorite buttermilk substitutions for baked goods (each is the equivalent of 1 cup of buttermilk):
¾ cup whole milk + 2 tablespoons heavy whipping cream + 1 tablespoon distilled white vinegar or fresh lemon juice