Preheat oven to 375°.
Prepare 18 muffin cups with cupcake liners OR cooking spray.
In a large bowl, combine flour, sugar, baking powder, baking soda, and salt.
In a separate small bowl, whisk together yogurt, eggs, and milk; add to flour mixture.
Stir in melted butter.
Toss blackberries in 1 Tbsp. flour; stir into batter.
Divide batter evenly among 18 muffins cups.
Prepare streusel topping and divide evenly among muffin cups.
Bake at 375° for 20-25 minutes.
Cool completely.
Combine ingredients for glaze and drizzle over cooled cupcakes.