1FOR THE TOPPING:
Stir together sugar and lemon zest in small bowl until combined; set aside.
2FOR THE MUFFINS:
Adjust oven rack to upper-middle position and heat oven to 425-degrees.
Spray standard muffin tin with nonstick cooking spray.
3Bring 1 cup blueberries, 1 teaspoon sugar and ½ teaspoon of lemon zest to simmer in small saucepan over medium heat. Cook, mashing berries with spoon several times and stirring frequently, until berries have broken down and mixture is thickened and reduced to ¼ cup, about 6 minutes. Transfer to small bowl and cool to room temperature, 10 to 15 minutes.
Soak 1 teaspoon of lemon zest into 1 tablespoon of lemon juice while the berries cool to room temperature, about 10 minutes.
4Whisk flour, baking powder, and salt together in large bowl. Whisk remaining 1 1/8 cups sugar and eggs together in medium bowl until thick and homogeneous, about 45 seconds. Slowly whisk in butter and oil until combined.
5Strain the 1 teaspoon of lemon zest of most it's moisture and add to the mixture -- discard the lemon juice. Whisk in buttermilk and vanilla until combined. Using rubber spatula, fold egg mixture and remaining cup blueberries into flour mixture until just moistened. (Batter will be very lumpy with few spots of dry flour; do not overmix.)
6Use ice cream scoop or large spoon to divide batter equally among prepared muffin cups (batter should completely fill cups and mound slightly).
7Spoon 1 teaspoon of cooked berry mixture into center of each mound of batter. Using chopstick or skewer, gently swirl berry filling into batter using figure-eight motion. Sprinkle lemon sugar evenly over muffins.
8Bake until muffin tops are golden and just firm, 15 to 19 minutes, rotating muffin tin from front to back halfway through baking time. Cool muffins in muffin tin for 5 minutes, then transfer to wire rack and cool 5 minutes before serving.