Place blackberries, lemon juice, water, and sugar into a pot and bring the heat to medium.
While it cooks, mash the berries. Cook until it thickens. Remember it will thicken as it cools.
If it's too thin, add a cornstarch slurry. If it's too thick, add more water.
Once ready, pour into a jar and cool in the fridge. It will last one week.