Preheat oven to 375 degrees. Line 12 standard muffin cups with paper liners. Whisk together flour, oat bran, baking powder, baking soda, cinnamon, and salt. In a large bowl, using a mixer, beat butter and 1/2 cup sugar on high until light and fluffy, about 6 minutes. Beat in eggs and vanilla until combined. With mixer on low, add flour mixture in 2 additions, alternating with buttermilk, and beat until combined. Fold in blackberries. Divide batter among muffin cups and sprinkle with remaining 1 tablespoon sugar. Bake until a toothpick inserted in center of a muffin comes out with moist crumbs attached, 20 to 25 minutes, rotating pan halfway through. Let cool in pan on a wire rack 5 minutes, then transfer muffins to rack and let cool completely. Cook's Note Store in an airtight container, up to 3 days.