Rinse the blueberries, then spread in a single layer on paper towels to dry. Pick over removing any stems or bad berries, then toss with flour. Set aside.
Preheat oven to 375°F. Line a 12 cup muffin tin with paper liners or spray with baking spray. Set aside.
Batter: In a medium size mixing bowl, use a whisk to sift together 2 cups flour, sugar, baking powder and salt.
In a separate bowl, whisk together buttermilk, oil, eggs and vanilla extract until fully combined.
Add the wet ingredients to the flour mixture. Stir by hand using a large wooden spoon or rubber spatula until combined. Fold in blueberries by hand. (You can use a hand mixer for the batter, if needed.)
Use an ice cream scoop or a 1/4 cup measuring cup to divide the batter evenly into each cup of the muffin pan.
Crumb Topping: Cut butter into sugar, oats and flour with a fork or a pastry blender until it resembles peas. Sprinkle evenly over the batter in each cup.
Bake for 25-30 minutes or until a toothpick inserted into the center comes back clean.
Cool in the pan for 3-5 minutes then remove to a wire rack to cool completely.
Store in an airtight container chilled in the fridge for up to 3-4 days or at room temperature for up to 2 days.