Combine blackberries and sugar in a bowl. Lightly mash the blackberries with a fork to release their juices. Cover and let sit overnight. Pour the mixture into an airtight container. Add the champagne vinegar and shake well. Store the container in the refrigerator for 3 days.
After 3 days give it a taste. It should be less sharp and the sweetness from the berries and sugar should stand out more. The vinegar will mellow more as it sits but it should always taste a bit like vinegar.