How to Make It
Combine 3 cups blueberries, water, sugar, lemon zest and juice, and vanilla in a medium saucepan. Bring to a boil over high; reduce to medium-low, and simmer until slightly thickened, about 20 minutes. Pour mixture through a fine wire-mesh strainer into a bowl, pressing gently on solids; discard solids. Let cool to room temperature. Reserve ½ cup blueberry sauce for Step
Remove the vanilla ice cream from the freezer, and thaw at room temperature 20 minutes. Stir together the cooled blueberry sauce and partially thawed ice cream in a large bowl, leaving some large pieces of frozen ice cream.
Whip the heavy cream to stiff peaks. Gently fold whipped cream into blueberry mixture, and swirl gently with a knife. Pour into a 9- x 5-inch loaf pan. Cover and freeze 6 hours or overnight.
To serve, run each side of loaf pan under hot water. Run a knife around the edge of the loaf pan, and turn out onto a cutting board. Cut into 3/4-inch-thick slices; place on serving plates. Top with remaining 1 cup blueberries and reserved 1/2 cup sauce; serve immediately.