Preheat the oven to 350F degrees and adjust the oven rack to the middle position. Lightly spray a donut pan with non-stick spray.
In a large mixing bowl, whisk together the flour, baking soda, salt, and cinnamon.
In a liquid measuring cup, combine the sour cream, brown sugar, melted coconut oil, egg, vanilla, and milk and whisk until well blended.
Using a rubber spatula, add the liquid ingredients to the dry ingredients and gently fold the mixtures together until just combined. Do not overmix. Gently fold in the blueberries.
Pour the batter into the prepared donut pan. Top each donut evenly with the prepared streusel topping. Gently press into the batter just enough so it sticks to the top of the batter.
Bake for 14 minutes, or until a toothpick inserted in a donut comes out clean. Cool for 2 minutes in the pan then move to a wire rack to cool completely.