Add the blackberries to a blender and pulse until fully pureed. You want a smooth, juicy consistency. Press the puree through a fine mesh sieve and discard the pulp. You should have about 2 cups of liquid. Transfer the liquid to a medium saucepan, add in the lemon juice and sugar. Heat over medium-high heat until reduced and thickened, about 5 minutes; stir occasionally. Transfer to a bowl and refrigerate until cold, about 2 hours. 1 Add the blackberries to a blender and pulse..
