1. Prepare the pan.
   Line an 8 × 8-inch pan with parchment paper, leaving enough overhang on the sides to lift out the finished fudge.
2. Cook the blueberry purée.
   In a small saucepan, combine the frozen wild blueberries, sugar, and lemon juice. Cook over medium heat for 5 to 7 minutes, stirring occasionally, until the berries soften and release their juices. Mash the berries as they cook.
3. Thicken the purée.
   In a small bowl, stir together the cornstarch and water to make a smooth slurry. Stir the slurry into the blueberry mixture. Cook for another 1 to 2 minutes, stirring constantly, until thickened.
4. Chill the purée.
   Remove from the heat and allow to cool. Refrigerate until completely chilled. Measure ⅓ cup for the fudge mixture and reserve the remaining purée for swirling.
5. Make the white chocolate fudge.
   In a medium saucepan, combine the white chocolate chips, sweetened condensed milk, and butter. Cook over low heat, stirring constantly, until the chocolate and butter are completely melted and the mixture is smooth. Do not let the mixture boil.
6. Add the blueberry purée.
   Remove the saucepan from the heat and stir in ⅓ cup of the chilled blueberry purée until evenly combined.
7. Assemble the fudge.
   Spread the white chocolate-blueberry mixture evenly into the prepared pan. Dollop the remaining chilled blueberry purée over the top. Drag a knife or skewer through the mixture to create a swirl pattern.
8. Chill and cut.
   Refrigerate for at least 2 hours, or until completely firm. Lift the fudge from the pan using the parchment paper and cut into small squares.

Store in an airtight container in the refrigerator.