Step 1: For the crust, combine graham cracker crumbs, ¼ cup sugar, and melted butter in a medium bowl. Press firmly into the bottom of a 9-inch springform pan. Freeze for 15 minutes.
Step 2: For the mango layer, take 2 cups of softened vanilla ice cream and gently fold in the mango puree until just combined. Spread this mixture evenly over the chilled graham cracker crust. Return to the freezer and freeze for at least 2 hours, or until firm.
Step 3: For the blueberry swirl, combine blueberries, 1 tbsp sugar, and lemon juice in a small saucepan. Simmer over medium heat for 5-7 minutes, mashing occasionally, until berries burst and form a thick sauce. Let cool completely.
Step 4: Take the remaining 2 cups of softened vanilla ice cream and gently swirl in the cooled blueberry sauce, leaving visible streaks. Carefully spread this blueberry ice cream mixture over the firm mango layer.
Step 5: Cover the springform pan tightly with plastic wrap. Freeze the mango blueberry ice cream cake for at least 6-8 hours, or preferably overnight, until completely solid.
Step 6: To serve, remove the cake from the freezer and let it sit at room temperature for 10-15 minutes. Run a thin knife around the edge of the pan before releasing the springform. Garnish with fresh mango cubes and whole blueberries. Slice with a hot, dry knife for clean cuts.