1. In a medium mixing bowl, beat egg lightly.
2. Stir in milk, oil, sour cream, and mashed bananas.
3. In a separate bowl, combine flour, sugar, baking powder, and salt.
4. Add all at once to liquid mixture and stir just until moistened.
5. Gently fold in blueberries (If frozen, toss in a small amount of flour first to coat)
6. Grease 2 muffin tins. Fill each cup 3/4 full.
7. Prepare streusel mixture by combining flour, sugar, and melted butter.
8. Top muffin batter with streusel mixture and press down gently.
9. Bake at 400 degrees for 20-25 minutes or until center are set and edges are beginning to be.
10. Remove from pan; cool completely.