Preheat oven to 375°F. Grease a 12-count muffin tin or line with paper liners. Set aside.
Whisk together oil and sugar until fluffy.
⅓ cup vegetable oil, 1 cup granulated sugar
Whisk in sour cream, buttermilk, egg, and vanilla until just combined.
½ cup sour cream, ½ cup buttermilk, 1 large egg, 1 teaspoon pure vanilla extract
Add in 1½ cups flour, salt, and baking powder and stir. Do not overmix.
1½ cups all-purpose flour, ½ teaspoon kosher salt, 2 teaspoons baking powder
Toss fresh blueberries with 2 teaspoons of flour until all the blueberries are coated.
1½ cups fresh blueberries
Add the blueberries into the mixture and gently fold until incorporated, making sure not to break any of the berries.
Spoon blueberry mixture into the prepared muffin tin until each well is ⅔ full.
Prepare brown sugar topping by stirring together brown sugar, flour, and melted butter.
½ cup brown sugar, ¼ cup all-purpose flour, 2 tablespoons unsalted butter
Sprinkle each muffin with a generous coating of brown sugar topping.
Bake in oven for 20 to 25 minutes, or until muffins spring back when touched or toothpick comes out clean.
Let muffins cool on a wire rack.