Prepare the Blueberry Compote: In a small saucepan, combine the blueberries, ¼ cup of sugar, cornstarch, and water. Set the pan over medium-low heat. Stir occasionally while the berries begin to break down and release their juices, which should take around 2–3 minutes. Once softened, use a potato masher to gently crush some of the berries. Continue stirring as the sauce simmers and thickens, about 2 more minutes. Remove from heat, let cool, then refrigerate until fully chilled.
Mix the Lemon Base: In a large mixing bowl, whisk together the sweetened condensed milk and lemon juice until the mixture is smooth and slightly thickened. Stir in the lemon zest for a vibrant citrus punch. Add the heavy cream, milk, sugar, salt, and vanilla extract. Whisk everything thoroughly until fully blended and the sugar has mostly dissolved.
Churn the Ice Cream: Pour the lemon mixture into your pre-chilled ice cream maker and churn according to your machine’s manufacturer instructions. This typically takes 20–25 minutes, or until the texture resembles soft-serve.
Layer with Blueberry Sauce: Once churning is complete, spoon some of the lemon ice cream into a freezer-safe container. Drizzle a portion of the cold blueberry sauce on top. Repeat this layering process, gently swirling with a knife or spoon for a marbled effect. Continue until all the ice cream and blueberry sauce are used.
Freeze Until Firm: Cover the container with a lid or plastic wrap and freeze for at least 3–4 hours, or until the ice cream is firm and scoopable. Serve and enjoy!