Step 1: Prepare the Blueberry Topping

1. In a small saucepan, combine the frozen blueberries, granulated sugar, and lemon juice over medium heat.
2. Stir frequently, allowing the blueberries to release their juices. Cook for about 5 minutes until bubbling.
3. In a small bowl, dissolve the cornstarch in 1 tablespoon of water. Add the mixture to the saucepan and stir until the sauce thickens and becomes glossy.
4. Remove from heat and let the blueberry topping cool completely.

Step 2: Make the White Chocolate Fudge

1. Line an 8×8-inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.
2. In a microwave-safe bowl, combine the white chocolate chips and sweetened condensed milk.
3. Heat the mixture in 30-second intervals, stirring in between, until the chocolate is completely melted and smooth.

Step 3: Add the Blueberry Swirl

1. Pour the white chocolate fudge mixture into the prepared pan and spread it evenly using a spatula.
2. Drop spoonfuls of the cooled blueberry topping onto the fudge.
3. Use a toothpick or knife to swirl the blueberry topping into the fudge, creating a marbled effect.

Step 4: Chill and Set

1. Let the fudge cool at room temperature for about 20 minutes.
2. Transfer the pan to the refrigerator and chill for 4–6 hours, or until the fudge is firm and fully set.

Step 5: Slice and Serve

1. Once set, lift the fudge out of the pan using the parchment paper. Place it on a cutting board.
2. Use a sharp knife to cut the fudge into small squares. Wipe the knife clean between cuts for neat edges.
3. Serve immediately or store in an airtight container.