Preheat the oven to 350F/180C.
In a large bowl mix the flour, sugar, baking powder, baking soda and salt and cut butter in with a pastry blender.
In a small bowl mix lightly beaten eggs, yogurt, vanilla and milk.
Add the wet ingredients to the dry ones.
Add blackberries. Stir gently in order not to crush blueberries.
Make the streusel topping by mixing all the ingredients with a fork.
Scoop into greased or paper lined muffin tin, sprinkle with a pinch of the streusel topping and bake at 350F/180C for 20-25 minutes.