Melt butter for batter and set aside. Toss blueberries in just a bit of flour so they are coated so they won't sink to the bottom. Shred zucchini over a few paper towels. Fold paper towels over and gently press down to remove excess water, set aside.
Combine all dry ingredients for batter in a bowl (leave topping for later), then add milk, butter and egg.
Once mixed well gently fold in your shredded zucchini and whole blueberries.
If you want to add the streusel topping combine ingredients for that well in a bowl. Cut in cold butter to create a crumble texture.
Grease muffin pan or use liners. Fill 3/4 of the way with batter. Sprinkle topping on top of a few or all of them, gently pat down.
Bake for 18-20 minutes, check after 18. Mini size takes about 11 minutes. Use a toothpick to check center, if it comes out mostly clean with crumbs on it remove so as not to over-bake them.