Place blackberries, lemon juice and sugar in a large saucepan.
Cook over medium heat, stirring frequently until it comes to a boil.
Continue to boil and stir for 10 minutes.
If you desire a thicker sauce, make a slurry of the cornstarch and water and add to boiling berries. Cook until thickened.
Remove from heat and cool. Store in the refrigerator until ready to use.
