Preheat oven to 190 C degrees and line muffin/cupcake pan with paper liners and set aside.
For crumb topping: In small bowl mix flour, sugar, and butter with a fork until crumbly. Refrigerate.
For muffins: In a large bowl stir together flour, baking powder and set aside. In a medium bowl whisk together eggs and sugar. Whisk in kefir/yogurt, oil, lemon and vanilla extract. Fold wet ingredients into dry ingredients and whisk everything together.
Place blueberries in a small bowl. Dust with 1 tablespoon flour and toss them until all blueberries are coated and fold them really gently in the batter.
Divide batter into prepared muffin tin. Cover with topping. Bake for 20 minutes or until the toothpick inserted in the center comes out clean.
Cool for 10 minutes in the pan then remove muffins and cool on a wire rack.