In a medium saucepan, combine fresh blueberries, sugar, and lemon juice over medium heat. Cook for about 5-7 minutes until the mixture starts bubbling, mashing the berries gently to release their juices. In a small bowl, whisk cornstarch with water until smooth, then pour it into the bubbling blueberry mixture, stirring continuously until thickened, about 2-3 minutes. Strain the sauce to remove solids and let it cool down to room temperature.

Preheat your oven to 325°F (165°C). In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter until well-blended and crumbly. Firmly press this crumb mixture into the bottom of a lined muffin pan, creating a solid base. Bake for 5 minutes until lightly golden, then let them cool completely before adding the filling.

In a large mixing bowl, beat softened cream cheese and sugar together until smooth and creamy, about 2-3 minutes. Gradually mix in fresh lemon juice, lemon zest, and vanilla extract until fully combined. Incorporate large eggs one at a time, ensuring not to overmix the filling.

Distribute the cheesecake filling evenly into each cooled crust, filling them about three-quarters full. Spoon the cooled blueberry swirl over the top of each cheesecake and swirl with a toothpick.

Place the assembled mini cheesecakes in the preheated oven and bake at 325°F (165°C) for 17-20 minutes or until the edges are set but the centers remain wobbly. Let them cool on a wire rack for about 1 hour.

Cover the mini cheesecakes with plastic wrap and refrigerate for at least 3-4 hours, or overnight for best results.