Prepare Blueberry Sauce: In a saucepan over medium heat, combine fresh blueberries with sugar and lemon juice. Cook while stirring gently for about 5 minutes. Mix cornstarch with water until smooth, then stir into the blueberries to thicken. Strain and set aside to cool.

Make Crust: Preheat your oven to 350°F (175°C). Combine graham cracker crumbs with melted butter and sugar until crumbly. Press about a tablespoon of the mixture into each muffin cup. Bake for 5 minutes until golden and cool slightly.

Prepare Cheesecake Filling: In a large bowl, beat cream cheese and sugar until smooth, about 2 minutes. Gradually add lemon juice, zest, and vanilla. Add eggs one at a time on low speed until smooth, taking care not to overmix.

Assemble Cheesecakes: Spoon filling into each crust about 3/4 full. Drizzle teaspoon of blueberry sauce on top and swirl with a toothpick to create a marbled effect.

Bake: Place muffin pan in the preheated oven at 325°F (160°C). Bake for 17 to 20 minutes until edges are set with a slight jiggle in the center. Turn off oven, leave cheesecakes inside for 10 minutes.

Cool and Chill: Let cheesecakes cool at room temperature for an hour. Transfer to the refrigerator to chill for 3 to 4 hours.