In a medium pot, combine 1 cup water, ¾ cup sugar, and 6 oz blueberries. Cook on medium-high 7 min., until blueberries burst, stirring occasionally. With a potato masher, mash berries in pot. Let mixture cool. Strain mixture through a fine-mesh sieve into a pitcher, pushing on solids. To pitcher, stir in 4 cups water and 1¼ cups fresh lemon juice. Refrigerate until cold. Serve over ice.