Preheat the oven to 400°F (204°C). Line a standard muffin or cupcake pan with paper liners. Set the prepared pan aside until you’re ready to fill it with batter.
In a small bowl, whisk together 1 cup all-purpose flour, 1/2 cup white sugar, and 1/2 teaspoon ground cinnamon (add more if you like a stronger cinnamon flavor). Pour in 6 1/2 tablespoons melted unsalted butter and stir with a fork until the mixture becomes crumbly. Place the crumb mixture in the refrigerator until you’re ready to use it. I find refrigerating the streusel helps it stay in nice, chunky pieces when baked.
In a large bowl, whisk together 1 1/2 cups all-purpose flour, 2 teaspoons baking powder, and 1/4 teaspoon salt, then set this bowl aside. In a separate medium bowl, place 2/3 cup white sugar and 1 tablespoon finely grated lemon zest; rub them together for a minute to infuse the sugar with lemon flavor. Add 2 large eggs and whisk until combined. Then whisk in 2/3 cup Greek yogurt, 1/3 cup vegetable oil, 2 tablespoons freshly squeezed lemon juice, and 1 teaspoon pure vanilla extract. The mixture should be pale and yellow.
Fold the wet ingredients into the dry ingredients just until combined; avoid over-mixing. Reserve 1/2 cup blueberries for topping. Place the remaining 2/3 cup + 1/6 cup (about 5/6 cup) blueberries in a small bowl. Add 1 tablespoon all-purpose flour and toss to coat the berries - this helps prevent sinking. Gently fold the floured blueberries into the batter, being careful not to crush them.
Spoon the batter into the prepared muffin tins, filling each cup no more than 2/3 full. Tap the pan gently on the counter to settle the batter. Top each muffin with the reserved 1/2 cup of blueberries, then generously add the refrigerated streusel crumb topping from Step 2 over the blueberries.
Place the pan in the oven and bake at 400°F (204°C) for 5 minutes. Without opening the oven, reduce the temperature to 375°F (190°C) and bake for an additional 13-15 minutes, until a toothpick inserted in the center comes out clean.
Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to finish cooling. I like to enjoy one while it’s still warm for the best flavor and texture!