Preheat oven to 350. Grease two standard muffin tins.
In a mixing bowl, beat together the eggs, oil, vanilla, sugar, zucchini, and lemon zest until well combined. Mix in dry ingredients until incorporated. Fold in blueberries. Scoop into prepared muffin tins (I used a 1/4 measuring cup for this and filled the cups almost completely full to get the oversized muffin effect).
In another smaller bowl, mix flour, sugar, salt, and softened butter together using your fingers or a pastry blender until it is crumbly. Sprinkle evenly over the tops of each muffin.
Bake 25-30 minutes or until muffins are golden and cooked through. Enjoy warm or at room temperature.