Preheat the oven to 325. Prepare four mini loaf pans or a regular size loaf pan by spraying with cooking spray. In a large bowl or the bowl of a stand mixer, beat together the sugar and butter until light and fluffy. Mix in the eggs, vanilla, and milk. Add the flour, salt, baking soda, and baking powder, and mix until just combined. Fold in the mashed bananas, then gently fold in the blueberries. Pour the batter into the prepared loaf pans.

To make the streusel, whisk together the sugar, flour, and cinnamon. Smash in the butter using your fingers, until the mixture resembles damp sand. Sprinkle the streusel mixture over the top of the loaves. Bake 35-40 minutes for mini loaves, or 60-70 minutes for a full size loaf, or until golden brown on top and a toothpick inserted into the center comes out clean. Let cool before serving.