Heat frozen blueberries, sugar, and lemon juice over medium for 5–7 minutes, stirring and mashing until softened.
Mix cornstarch with 1 tablespoon cold water. Stir into the fruit and cook for 1–2 minutes until the juice thickens.
Cool in a shallow bowl, then refrigerate until completely cold, allowing about 1 hour. Measure 1/3 cup and reserve the remaining puree.
Make and Set the Fudge
Stir white chocolate chips, condensed milk, and butter over low heat for 3–5 minutes until melted and smooth. Remove from heat without boiling.
Blend in the measured 1/3 cup puree and spread in a parchment-lined 8-inch square pan. Dollop the reserved puree on top and draw a skewer through it to form swirls.
Refrigerate covered for at least 2 hours until the center is firm. Lift out and cut into 16 equal squares; serve chilled.
Notes
Keep refrigerated in a covered container and use within 5 days.
Freeze for up to 1 month; thaw cold and expect a softer swirl.