Make the simple syrup: Combine water and sugar in a small saucepan over medium heat. Stir until sugar fully dissolves (3–5 minutes). Cool slightly.
Purée the blueberries: Blend until completely smooth. For the smoothest texture, strain through a fine-mesh sieve to remove skins and any seeds (press firmly to extract as much purée as possible).
Combine: Mix the strained purée, cooled syrup, and lemon juice. Taste and add more sugar or lemon if needed. Cover and chill in the fridge for 1–2 hours (or overnight for best results).
Churn: Ensure your Cuisinart freezer bowl is fully frozen. Turn on the machine, pour in the chilled mixture, and churn for 15–25 minutes until thick and creamy.
Serve or store: Enjoy soft-serve style immediately, or transfer to an airtight container and freeze 2–4 hours for firmer scoops. Soften at room temp for 10 minutes before serving.