Combine the blueberries, sugar, lemon juice, vanilla and 3/4 cup water in a blender and blend until smooth. Pour the liquid through a fine-mesh sieve into a 9-inch square glass baking dish, pressing hard on the solids with a spatula before discarding. Freeze the mixture, stirring and crushing up clumps of ice with a fork every hour, until it is frozen, but slushy, about 4 hours total. Spoon into glasses and garnish with the mint or basil leaves.