Preheat oven to 400 F (204 C).
Using a fine mesh strainer, sift flour, baking power, baking soda, and salt into a large bowl. Set aside.
1 cup all-purpose white flour, ½ teaspoon baking powder, ⅛ teaspoon baking soda, ¼ teaspoon salt
Add buttermilk, sugar, vanilla, egg, and butter to a blender and blend on low speed for about 30 seconds. Alternatively, you can add these ingredients to a medium bowl and whisk or mix with an electric mixer until frothy.
¼ cup buttermilk, ½ cup granulated sugar, ½ teaspoon vanilla extract, 1 large egg, 1 tablespoon unsalted butter
Pour wet ingredients into dry and stir until no flour streaks remain. Do not overmix.
Add ¼ cup of the blueberries to a small bowl and carefully crush with the back of a fork. Take care because the juice tends to splatter. If using frozen blueberries, measure out a little more than ¼ cup, thaw, then crush.Stir crushed blueberries into the batter.
½ cup blueberries
Gently fold remaining blueberries into the batter.If using frozen berries, keep frozen. Spoon mixture into a piping bag fitted with a large round tip (tip #1A - about ½ inch diameter).If you don't have a piping bag and tip, you can instead add the batter to a gallon zip lock food storage bag, seal it, and snip off one corner with scissors.
Spray donut pan with baking spray. Pipe batter into donut cavities, filling each about ¾ full. Two turns inside each cavity should be enough.
Bake for 20-25 minutes or until a toothpick inserted toward the center comes out clean. Oven temperatures vary so begin to check for doneness at 20 minutes.
Let cool in pan for several minutes. Then invert donuts onto a wire rack to cool.