Mash blackberries with sugar. Let sit 24 hours in the fridge. You can let it sit up to 3 days.
Strain the blackberry mixture. Add red wine vinegar. Whisk to combine until sugar is dissolved. Pour into a jar and store in the fridge.
Notes
To use: pour a splash to several tablespoons in an 8 oz. glass and top with club soda, or use in a mixed drink.This will last several months in an air tight container in the fridge.