Preheat oven to 375 degrees. Line a 12-cup muffin pan with paper liners. In a bowl whisk together whole-wheat flour, all-purpose flour, 1/2 cup toasted wheat germ, baking powder, salt, and 2/3 cup brown sugar. Make a well in center of bowl and add milk, vegetable oil, eggs, and vanilla extract. Gently mix until just combined. Fold in blueberries.
Divide batter among muffin cups. Combine 2 Tbsp. toasted wheat germ, 1 Tbsp. brown sugar, and rolled oats in a small bowl. Sprinkle over tops of muffins. Bake until a toothpick inserted in center comes out clean, 20 to 22 minutes. Let sit in pan 10 minutes. Transfer muffins to a rack to cool.