Add the blueberries, granulated sugar, and lemon juice to a small saucepan. Cook over medium heat, stirring occasionally, until the berries start to burst and the juices look syrupy, about 5–7 minutes. Stir the cornstarch and water mixture, then pour it into the berries while stirring. Cook 1–2 more minutes, until the mixture thickens slightly and coats the back of a spoon. Remove from heat and let cool to room temperature; it will thicken more as it cools.

Line an 8x8-inch (20x20 cm) baking pan with parchment paper, letting the paper hang over the edges to create handles. Lightly spray the parchment with nonstick spray or rub with a small amount of butter to make slicing easier.

In a medium saucepan, add the white chocolate chips, sweetened condensed milk, and butter. Cook over low heat, stirring constantly with a heatproof spatula, until the mixture is fully melted and smooth. Keep the heat low so the white chocolate does not scorch or seize. Remove from heat and quickly whisk in the softened cream cheese, vanilla extract, and salt. Stir until the cream cheese is completely blended and the mixture looks thick and glossy.

Pour the cheesecake fudge base into the prepared pan and smooth the top with an offset spatula or the back of a spoon. Spoon small dollops of the cooled blueberry mixture over the surface, about 8–10 dollops spread across the top. Use a butter knife to gently swirl the blueberry sauce through the fudge, dragging the knife in soft, curving motions without over-swirling so you keep distinct streaks of color.

Cover the pan loosely with plastic wrap or foil. Chill in the refrigerator for at least 3–4 hours, or until firm; overnight gives the cleanest slices. Once set, lift the fudge out using the parchment handles and place on a cutting board. Use a sharp knife, wiping the blade between cuts, to slice into 16–25 squares, depending on your preferred size.