For the streusel:
1/2 cup packed light brown sugar
1/2 cup rolled oats (not instant)
1/3 cup all-purpose flour
1/4 teaspoon fine salt
6 tablespoons cold unsalted butter (3/4 stick), cut into small pieces
For the cake:
2 cups fresh blueberries (about 10 ounces)
2 cups plus 2 tablespoons all-purpose flour
1 tablespoon baking powder
1 teaspoon ground cinnamon
1/2 teaspoon fine salt
1/4 teaspoon ground nutmeg
2/3 cup milk (not nonfat), at room temperature
1 teaspoon vanilla extract
8 tablespoons unsalted butter (1 stick), at room temperature, plus more for coating the pan
1/2 cup plus 2 tablespoons packed light brown sugar
1/2 cup plus 2 tablespoons granulated sugar
2 large eggs, at room temperature
For the streusel:
Mix the brown sugar, oats, flour, and salt in a medium bowl until evenly combined. With your fingertips, blend in the butter pieces until small clumps about the size of a dime form and the butter is well incorporated, about 2 minutes. Refrigerate the streusel while you make the cake.
For the cake:
Heat the oven to 350°F and arrange a rack in the lower third. Coat a 10-inch springform pan with butter. Line the bottom of the pan with parchment paper, coat the parchment with butter, and set the pan aside.
Rinse the blueberries and dry off any excess water (but leave the berries slightly damp). Transfer to a medium bowl, add 2 tablespoons of the flour, and toss to coat; set aside.
Whisk the remaining 2 cups flour, baking powder, cinnamon, salt, and nutmeg together in a medium bowl until evenly combined; set aside. Stir the milk and vanilla together in a small bowl or measuring cup until evenly combined; set aside.
Place the measured butter in the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed until smooth and light in color, about 2 minutes. Add the sugars slowly, about 2 tablespoons at a time, until both have been added, about 5 minutes total. Stop the mixer and scrape down the sides of the bowl and the paddle with a rubber spatula. Return the mixer to medium speed and add the eggs one at a time, letting the first completely incorporate before adding the second. Scrape down the sides and paddle again.
Reduce the speed to low, add a quarter of the flour mixture, and mix until just combined. Pour in a third of the milk mixture and mix until just incorporated. Repeat, alternating dry and wet ingredients and ending with the dry ingredients, until completely incorporated. (Do not overmix or the cake will be tough.)
Remove the bowl from the mixer, add the reserved blueberries (leaving any excess flour behind), and, using a rubber spatula, fold in the berries until evenly distributed. Turn the batter into the prepared pan. Sprinkle the chilled streusel evenly over the batter and lightly press it in with your hands. Bake until a tester inserted into the center of the cake comes out clean and the top is golden brown, about 55 to 60 minutes.
Remove the pan from the oven and let it cool on a wire rack for at least 20 minutes. If the sides of the cake have not pulled away from the pan by then, carefully run a knife around the outer edge of the cake. Unlock and remove the outer ring. Let the cake cool for 30 minutes more.
To remove the bottom of the pan, place a large piece of foil over the top of the cake and gently fold it around the sides. Place a cooling rack or plate on top and invert the cake. Remove the springform bottom and peel off the parchment paper, then place a cooling rack or serving dish on top of the cake. Invert the cake again, remove the foil, and serve. The cake can be stored in an airtight container at room temperature for up to 2 days.