For the topping: Mix the sugar and lemon zest together in a small bowl. Set aside.
Place the baking rack in the center of the oven. Preheat the oven to 425°F (220°C). Line a muffin pan with paper or silicone muffin cups.
In a small saucepan, simmer 2 cups (500 mL) of blueberries and 2 teaspoons (10 mL) of sugar over medium heat. Lightly crush the blueberries with a spoon. Simmer, stirring frequently, until all the blueberries are crushed and the mixture has thickened (about 6 minutes). Transfer the blueberry mixture to a bowl and let stand at room temperature for about 10 to 15 minutes.
In a large bowl, combine the flour, baking powder, and salt.
In another bowl, whisk the sugar with the eggs. Gradually add the butter and oil until the mixture is homogeneous. Then add the yogurt and vanilla extract.
Add the egg mixture and the remaining blueberries to the dry ingredients. Mix to incorporate all the flour.
Fill the muffin cups half-full. Add about 1 tablespoon (15 mL) of the blueberry mixture to each cup. Using a toothpick, swirl the two mixtures together. Divide the remaining muffin batter among the cupcake cups. Finally, add the remaining blueberry mixture and cover with the topping.
Bake for 17 to 20 minutes, or until a toothpick inserted in the center of a muffin comes out clean. Let cool before serving.
