For the blueberry puree: Place the fruits in a large bowl, then add the sugar or your choice and the lemon juice. Mix to coat the fruits thoroughly. Let rest at room temperature for 15 to 30 minutes. If the blueberries you’re using are frozen, let rest for 45 to 60 minutes to give the fruits time to thaw completely, then macerate.
Transfer the macerated blueberries to a saucepan. Place over medium-high heat and bring to a boil. Lower the heat and simmer until the blueberries have broken down and the syrup has thickened, 8 to 10 minutes. If the blueberries you used were frozen, the simmering time could take up to 15 minutes before excess water evaporates and the mixture thickens. Remove from the heat and let cool to room temperature. Transfer to an airtight container and refrigerate at least 2 hours or overnight.
Prepare the gelato base of your choice according the recipe instructions. Refrigerate until completely cold.
To churn: Transfer the fruit mixture to a blender and mix until you get a fine puree consistency (you can also use a stick blender to do this). Measure out 2 cups (500 ml) of blueberry puree for the gelato (save the remaining puree, if any, to spoon over the churned gelato as a coulis, or add to a smoothie.) Whisk the blueberry puree into the prepared gelato base. Pour into the bowl of an ice cream maker and churn according to the manufacturer's instructions. Stop the machine when the gelato is icy but still soft.
Transfer the gelato to an airtight container and freeze until firm, about two hours. The gelato will keep, frozen, for up to two weeks. Always take the gelato out to room temperature 10 to 15 minutes before serving to soften it and make it easier to scoop.