Rinse the blackberries and pat dry. You can cut larger berries in half.
In a small saucepan combine the sugar, lemon zest and blackberries and cook on medium heat for 8-10 minutes or until the berries soften.
Mix the lemon juice and water in a small measuring cup or glass. Whisk in the cornstarch to make a paste.
Pour the thickener into the blackberries and stir occasionally until the mixture begin to thicken, about 4-5 minutes.
Transfer sauce into a mason or jelly jar and let cool.The sauce will continue to thicken as it cools.
Notes
Choose Fresh, Ripe Blackberries: Fresh and ripe blackberries are naturally sweet and flavorful, making them the best choice for your sauce. However, if fresh blackberries aren't available, frozen ones can be a good substitute.Add Sweetener: Depending on the sweetness of your blackberries and your personal taste, you might want to add some sweetener. Sugar is the most common choice, but honey or maple syrup can also be used for a different flavor profile.Use the Right Amount of Liquid: A little bit of liquid can help to prevent the sauce from becoming too thick. Water, fruit juice, or even a splash of wine can be used. Keep in mind that the blackberries will also release some juice as they cook.Cook Slowly: Cook the blackberries over medium-low heat until they break down and release their juices. This can take about 10 minutes.Strain if Desired: If you prefer a smoother sauce without the blackberry seeds, you can strain the sauce through a fine-mesh strainer. However, this step is entirely optional and a matter of personal preference.Add a Thickener if Needed: If your sauce is too thin, you can add a slurry made from a little cornstarch and water to help thicken it. Just be sure to cook the sauce for a few minutes after adding the slurry to get rid of the starchy taste.Adjust to Taste: Once your sauce is cooked, taste it and adjust the sweetness or acidity as needed. A little bit of lemon juice can help to balance out the sweetness and bring out the flavor of the blackberries.Cool Before Using: Let the sauce cool before using it. As it cools, it will thicken slightly.Recipe can be doubled.