Preheat your oven to 400°F and grease a 9-cup muffin tray or use paper liners.
In a medium bowl, whisk together the Bisquick and sugar.
In a liquid measuring cup, measure ⅔ cup of milk. Add the egg and oil and whisk until evenly incorporated.
Pour the liquid ingredients into the dry ingredients and gently mix until JUST combined; there should be plenty of lumps in the batter. Fold in the blueberries.
Divide the batter into the 9-cup muffin tray. If your tray holds more than 9 muffins, add a tablespoon of water to any empty cups.
Optional streusel: Combine the streusel ingredients into a small bowl and use a pastry cutter to work in the butter until it resembles coarse bread crumbs. Sprinkle the streusel on top of the muffins.
Bake for 15-18 minutes (15 for fresh blueberries and 18 for frozen blueberries) until golden brown on top.