Prepare a muffin pan by placing paper liners in each cup.
In a large bowl mix the melted butter and the sugar together with a whisk until well combined.
Add the eggs, lemon zest, and vanilla extract to the sugar mixture and continue whisking until the mixture lightens in color and is well blended.
In a small bowl combine the flour and baking powder and mix together.
Add half of the flour mixture to the egg mixture, followed by half of the milk. Whisk together until well combined. Add the remaining flour mixture and milk to the bowl whisking until everything is well combined and the batter is smooth.
Gently fold in the blackberries.
In a separate bowl combine the lemon crumb topping ingredients and using a fork mix then together until they are crumbly.
Divide the batter equally among the muffin tin filling each one approximately ¾ full. See note #2
Sprinkle each muffin with the lemon crumb topping.
Bake for 25 - 30 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Once they have finished baking remove from the oven and let cool in the pan for 15 minutes. Then remove from the pan and let them cool completely before storing in an airtight container
Notes
If you use frozen berries thaw them completely and then pat dry with a paper towel. This will help remove the excess juice and keep the batter from becoming colored with the juices.
The amount of batter used may vary depending on the size of your blackberries. If you have a little extra batter left at the end you can bake a second batch of muffins or dispose of it. If you bake a pan of muffins that has some unused cups fill each one with ½ inch of water before baking to help the muffins bake evenly.