Preheat oven to 325°F (163°C). Line a standard 12-cup muffin pan with paper liners. In a medium bowl, mix the graham cracker crumbs and 1/4 cup sugar. Pour in the melted butter and stir until the mixture resembles wet sand. Press about 1.5 tablespoons of the mixture firmly into the bottom of each muffin cup. Bake for 5 minutes, then remove and let cool slightly while you prepare the filling.
In a small saucepan over medium heat, combine the blueberries, 2 tbsp sugar, cornstarch, water, and 1 tsp lemon juice. Cook, stirring frequently and mashing the berries slightly, until the mixture thickens and becomes jam-like, about 5-7 minutes. Remove from heat, let cool for 10 minutes, then strain through a fine-mesh sieve to remove skins if desired. Set aside to cool completely.
In a large bowl using a hand mixer or stand mixer with the paddle attachment, beat the softened cream cheese on medium-high speed until completely smooth and creamy, about 2 minutes. Add the 2/3 cup sugar and beat until combined. Add the eggs one at a time, beating on low speed after each addition until just incorporated. Scrape down the sides of the bowl. Add the sour cream, 1/4 cup lemon juice, lemon zest, and vanilla extract. Beat on low speed until the batter is smooth and uniform. Do not overmix.
Spoon or pipe the lemon cheesecake filling over the pre-baked crusts, filling each muffin cup nearly to the top. Dollop about 1 teaspoon of the cooled blueberry sauce onto the center of each cheesecake. Use a toothpick or skewer to gently swirl the blueberry into the lemon filling, creating a marbled effect.
Bake the cheesecakes for 18-22 minutes, or until the edges are set and the centers only have a slight jiggle. Remove from the oven and let the cheesecakes cool in the pan on a wire rack for 1 hour. Then, transfer the pan to the refrigerator to chill for at least 4 hours, or preferably overnight, before serving.
Carefully remove the chilled cheesecakes from the pan and peel away the liners. Garnish with fresh blueberries, a sprinkle of lemon zest, or a dollop of whipped cream if desired. Serve immediately.