Prepare Blueberry Sauce: In a saucepan, combine fresh blueberries, sugar, and lemon juice over medium heat. Gently mash the berries while cooking, and add a cornstarch-water mix. Cook until thickened, then strain and cool for a smooth sauce.

Preheat Oven: Set your oven to 325°F (160°C) to ensure it's ready for your mini cheesecakes.

Make Crust: In a bowl, mix graham cracker crumbs, granulated sugar, and melted unsalted butter. Press the mixture firmly into a muffin pan lined with cupcake liners. Bake for 5 minutes, then allow to cool completely.

Mix Filling: In a large bowl, beat the cream cheese and sugar until smooth and creamy. Gradually add lemon juice, zest, and vanilla extract. Incorporate eggs one by one on low speed, mixing until just combined to avoid overmixing.

Assemble: Divide the cheesecake filling among the cooled crusts, then add a spoonful of the blueberry sauce on top. Swirl gently with a toothpick or knife for a beautiful marbled effect.

Bake: Place the filled muffin pan in the oven and bake for 17-20 minutes. The tops should be set but still have a slight jiggle in the center.

Cool & Chill: Allow the cheesecakes to cool at room temperature for 1 hour. Then, refrigerate them for 3-4 hours or overnight to fully set.