1. In a small pot, bring the sugar and water just to a boil over medium heat. Stir until sugar dissolves. Remove from heat and let cool. Refrigerate until chilled.
2. Line a 9x5-inch loaf pan with plastic wrap so it overhangs on all sides.
3. In a large bowl stir the sorbet to soften. Spread evenly in the loaf pan. Freeze for 15 minutes. Wipe bowl clean. In the same large bowl, stir the vanilla ice cream to soften. Spread in a an even layer on top of the sorbet. Freeze 15 minutes.
4. In a food processor, combine the blueberries and lemon juice. Cover and pulse until the berries are finely chopped. Scrape down the sides of the bowl. Add the sugar syrup and puree the berries until they form a sorbet, stopping to scrape down the sides as needed. Immediately spread blueberry mixture over vanilla ice cream in the loaf pan and smooth the top.
5. Press the plastic wrap on the surface of the terrine and freeze for at least 6 hours or overnight. Using the plastic wrap, lift the terrine out of the pan onto a work surface. Slice the terrine and serve immediately.