In a medium bowl, whisk together the sugar, flours, oats, cinnamon, nutmeg, and salt. Cut in the butter using a pastry blender or two knives (or your hands) until you have medium-size crumbs. Set aside.
In a medium bowl, wish together the flours, baking powder, and salt.
In the bowl of the stand mixer, cream together the butter and sugar until light and fluffy. Add the eggs, one at a time, scraping down the sides and bottom of the between between.
Stir in half the dry ingredient, then add all the milk and the vanilla, scraping down the sides as you mix. Stir in the remaining dry ingredients. Gently fold in the blueberries.
Spread the batter into the prepared pan and sprinkle the streusel mix evenly over the top.
Bake in the preheated oven about 45 to 50 minutes, until a tester comes out clean. Remove from the oven and cool in the pan on a wire rack.