Heat oven to 375°F. Grease bottoms only of 18 muffin cups or line with paper baking cups. In large bowl, combine 1/2 cup margarine and 1/2 cup brown sugar; beat until fluffy. Add bananas, milk, vanilla and eggs; blend well.
Add 2 cups flour, baking powder, baking soda and salt; stir just until dry ingredients are moistened. Gently stir in blueberries. Fill greased muffin cups 3/4 full.
In small bowl, combine 3 tablespoons flour, 2 tablespoons brown sugar and cinnamon; mix well. With pastry blender or fork, cut in 2 tablespoons margarine until mixture is crumbly. Stir in walnuts. Sprinkle over batter.
Bake at 375°F. for 18 to 23 minutes or until toothpick inserted in center comes out clean. Cool 3 minutes; remove from pan. Serve warm.