In a skillet over medium heat, cook blueberries with lemon juice and ¼ cup sugar until berries soften, about 3 minutes. Remove from heat and set aside.
In a medium-size saucepan, bring cream and lemon zest to a simmer; then remove from heat. Meanwhile, beat egg yolks with remaining ½ cup sugar until pale and fluffy, about 3 minutes.
Pour ½ cup hot cream into egg mixture, whisking as you go; then transfer it all back into pot to create a loose custard. Stir in half of blueberries. Heat custard over medium heat, stirring continuously, until it reaches 175° on an instant-read thermometer (this is the point at which it will thicken noticeably).
Remove from heat, stir in sour cream and salt, and pour through a strainer into a bowl, mashing blueberries with back of spoon to release juices. Chill in refrigerator until very cold, 6–12 hours. For faster results, stir over a bowl of ice, but let custard sit several hours for the best texture.
Once mixture is chilled, it should register between 35° and 40° on an instant-read thermometer. Pour into ice-cream maker, leaving 1 inch at top to allow for expansion; prepare according to freezer instructions.
Add remaining berries during final 5 minutes of chilling. For firmer texture, chill in refrigerator at least 3 hours.