Preheat the oven to 425°F.
For crumble topping, mix melted butter, sugar, lemon zest and flour. Set aside.
For the batter, begin by sifting 2 1/2 cups flour into a medium-sized mixing bowl.
In a separate bowl, whisk melted butter, oil, eggs, and milk (or buttermilk).
Add flour, baking powder, and baking soda to the liquid mixture. Stir with a spatula until just combined. Don't over-mix.
Coat blueberries with flour.
Add blueberries to the batter. Fold in gently, making sure not to overmix.
Spoon into the muffin pans, filling each cup about two-thirds full.
Top with streusel mixture.
Bake at 425°F for 10 minutes. Lower the temperature to 375°F and bake for another 10-15 minutes.