Combine the blueberries, lemon juice, and cornstarch in a medium saucepan over medium heat. Bring the mixture to a simmer. Stir frequently and mash the blueberries gently as they cook. Continue simmering for 10–12 minutes, or until the mixture has thickened. Remove from heat.

(Optionally, strain the blueberry mixture through a fine-mesh sieve to remove solids.)

Reserve 3 tablespoons of the blueberry reduction in a separate bowl for swirling; set both portions aside.

Grease an 8- or 9-inch baking dish lightly with butter or nonstick spray. Line it with parchment paper, ensuring excess parchment hangs over the edges for easy removal.

In a medium saucepan over medium heat, combine the sweetened condensed milk and white chocolate. Stir continuously until the mixture is fully melted and smooth, approximately 8–10 minutes. Add the butter, vanilla extract, and kosher salt. Stir until the butter is fully incorporated, then remove from heat.

Add the blueberry reduction (excluding the reserved 3 tablespoons) to the fudge mixture. Stir only 1–3 times to create subtle streaks without fully blending the mixture.

Using a rubber spatula, pour the fudge mixture into the prepared baking dish and spread evenly.

Drop small spoonfuls of the reserved blueberry reduction onto the surface of the fudge. Use a toothpick or skewer to swirl the reduction into a marbled pattern.

Refrigerate the fudge for 2–3 hours, or until fully set. Once firm, lift the fudge from the baking dish using the parchment overhang. Slice into squares and serve.