Preheat oven to 350 ℉. Line a greased 9x5 loaf pan with parchment paper and set aside. Combine streusel ingredients in a small bowl and stir until combined. Set aside.
In a medium bowl, add the sugar and lemon zest. Rub together with your fingers until incorporated and fragrant. Add melted butter, extract and egg yolk. Mix until combined. Add the flour, baking powder, salt, buttermilk and lemon juice. Mix just until combined.
Spread half the batter into prepared loaf pan. Top with about ⅔ of the blackberries and half of the streusel. Gently spread remaining batter on top of the berries and streusel with the back of a spoon or offset spatula. Sprinkle remaining blackberries and streusel on top.
Bake for about 30-40 minutes or until a toothpick inserted into the center comes out without batter. Let cool in the pan for 10 minutes before removing to a cooling rack to cool most of the way. Make the glaze by stirring together the powdered sugar and lemon juice. Drizzle over the coffee cake and slice into 6 pieces.