In a medium saucepan, combine fresh blueberries, granulated sugar, and a splash of lemon juice over medium heat. Use a fork to mash the blueberries gently as the mixture cooks, bringing it to a bubbling simmer. After about 5 minutes, mix 1 teaspoon of cornstarch with a tablespoon of water, adding it to the pan to thicken your sauce. Once thick, strain the sauce to remove the skins and set aside to cool.

Preheat your oven to 325°F (163°C). In a mixing bowl, combine the graham cracker crumbs, sugar, and melted unsalted butter until well mixed. Line a muffin tin with paper liners, then firmly press this mixture into the bottom of each liner to create a solid crust. Bake in the preheated oven for about 5 minutes, until slightly golden, then remove and allow to cool completely.

In a large mixing bowl, beat the block cream cheese and sugar until smooth and creamy using an electric mixer. Add in the freshly squeezed lemon juice, lemon zest, and pure vanilla extract, mixing until well combined. Now, incorporate the eggs one at a time, mixing gently on low speed just until each egg is incorporated to avoid overmixing, which can create cracks in your cheesecakes.

Carefully divide the cream cheese filling among the cooled crusts, filling each cup about three-quarters full. Spoon a teaspoon of the prepared blueberry sauce onto each mini cheesecake, then use a toothpick or knife to swirl it gently into the filling, creating a beautiful marbled effect.

Place the muffin tin in the oven and bake for 17-20 minutes. You want the mini cheesecakes to be set around the edges but slightly jiggle in the center. Once baked, turn off the oven and leave the cheesecakes inside for an additional 10 minutes.

After the cheesecakes have cooled in the oven, remove them and allow them to cool at room temperature for about an hour. Then, transfer them to the refrigerator, covering them lightly, and chill for at least 3-4 hours or ideally overnight.