1. Adjust oven rack to upper-middle position and heat oven to 425 degrees. Spray standard muffin tin with nonstick cooking spray. Bring 1 cup blueberries and 1 teaspoon Splenda 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.
2. Whisk flour, baking powder, and salt together in large bowl. Whisk remaining 11/8 cups Splenda and eggs together in medium bowl until thick and homogeneous, about 45 seconds. Slowly whisk in butter and oil until combined. 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.)
3. Use ice cream scoop or large spoon to divide batter equally among prepared muffin cups (batter should completely fill cups and mound slightly). Spoon 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.
4. Bake until muffin tops are golden and just firm, 17 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.
I know most people are not a fan of Splenda and you can definitely make these without the use of Splenda, just substitute with your sweetener of choice (the measurements might be different though). I personally use Splenda because I can not eat Sugar. I am still in the process of trying other sugar alternatives but since I live in a rural area I have to order them over the Internet since they only sell Splenda at our Lil market.