Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together 2 ½ cups + 2 tablespoons of flour, 1 cup + ¼ cup sugar, and ¾ teaspoon cinnamon.
In a separate bowl, whisk together 2 eggs, ½ cup sour cream, 2 teaspoons vanilla extract, ¼ cup safflower oil, and ¾ cup whole milk.
Gently fold the wet ingredients into the dry ingredients. Do not overmix.
Carefully fold in the fresh blueberries.
For the streusel topping, mix ¼ cup granulated sugar, 3 tablespoons softened butter, ¼ cup flour, and 2 teaspoons cinnamon until crumbly.
Spoon the batter evenly into the muffin cups, filling them to the top. Sprinkle streusel topping over each muffin.
Bake at 425°F (218°C) for 7 minutes, then lower the temperature to 350°F (177°C) and continue baking for 22-25 minutes, until a toothpick inserted in the center comes out clean.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.