In a small mixing bowl, whisk together the flour, oats, brown sugar, and melted butter until the mixture comes together in a ball. Flatten the ball into a disc, place it back in the mixing bowl and place the bowl in the freezer while you prepare the muffin batter.

Preheat the oven to 350ºF and grease a 12 cup muffin pan or line it with paper cups. In a large mixing bowl, whisk together the oats, flours, and baking soda. In a separate mixing bowl, whisk together the melted butter, brown sugar, buttermilk, eggs, and vanilla extract until well combined.

Pour the wet ingredients into the bowl with the dry ingredients and fold together with a wooden spoon or spatula until just combined, being careful not to over-mix. Add the blueberries to the bowl with the batter and gently fold them into the batter with a rubber spatula or flat spoon.

Fill the prepared muffin cups about 3/4 full of batter. Remove the crumb topping from the freezer and crumble the disc into the bowl to create big, dry crumbs. Top each batter-filled muffin cup with about 2 Tablespoons of crumb topping. Bake the muffins in the preheated oven for 25-35 minutes, or until the tops are golden brown and a toothpick inserted in the center of a muffin comes out clean. Allow the muffins to cool at least 10 minutes in the pan before removing them and serving.