In a small saucepan, combine blackberries, sugar, and water. Bring to a gentle simmer over medium heat and cook for 5 minutes. Gently mash the berries with a spoon to release their juices, then simmer for 5 more minutes. Strain through a fine mesh sieve into a jar, pressing out all the syrup. Let cool before using. Store in the fridge for up to 1 week!