Start by making the puree. Combine blueberries, ⅓ cup of granulated sugar and the lemon juice in a medium saucepan over medium heat. Stir occasionally. The blueberries will release their juices and will slowly come to a boil. Take the blueberries off the heat once they reach a boil. Lightly mash the blueberries with a potato masher.
If you would like smooth ice cream, puree the blueberries in a blender. Pour the puree into a bowl, cover with plastic wrap, and cool in the refrigerator. If you would like tiny chunks of blueberries in the ice cream, pour the mashed blueberries into a bowl, cover with plastic wrap, and cool in the refrigerator. This can be done a day or two ahead.
To make the custard base, combine milk, 1 cup of heavy whipping cream, and salt in a medium saucepan over medium heat. You want to heat the milk mixture up until hot, but not boiling.
Meanwhile, pour the remaining 1 cup of heavy whipping cream into a medium size bowl and top it with a fine mesh strainer. Set aside.
Whisk the egg yolks in a small bowl and set aside.
When the milk mixture is hot, slowly ladle an amount into the egg yolks, whisking continuously to temper the eggs. Pour the tempered eggs into the saucepan with the rest of the milk mixture, and whisk constantly over medium heat. The mixture will be ready when it has thickened and coats the back of a spoon, or the temperature reaches 170°F.
Immediately strain the custard into the prepared cold heavy cream and stir to stop the custard from cooking. Place the bowl of custard over an ice bath and allow the custard to cool completely, about 30 minutes. Cover and refrigerate until ready to churn, 4 hours or overnight.
When you are ready to churn the ice cream, stir in the vanilla extract to the cooled custard base. Then stir in the blueberry puree. Transfer the ice cream to an ice cream maker and churn according to the manufacture's instructions.
Store the ice cream in an airtight container and place it into the freezer for several hours or until it is ice cream consistency.