Preheat oven to 400°. Grease and flour loaf pan or line with parchment paper.
Combine flour, sugar, baking powder, baking soda, and salt in a large bowl.
In a small bowl, whisk together eggs, milk, butter, and vanilla.
Pour wet ingredients into dry ingredients, and mix until just combined.
Fold in mashed blackberries.
Pour batter into prepared loaf pan.
Place flour, brown sugar, and butter for the topping in a small foor processor or mix by hand. Pulse or mix until all ingredients are combined, and mixture looks like coarse crumbs.
Evenly sprinkle topping over unbaked bread.
Bake at 400 degrees for 45-60 minutes, until the top is brown and a toothpick inserted in the center tests clean. Check bread after 40 minutes, and loosely cover with aluminum foil if the top is getting too brown.
