Prepare the Blackberries: In a large bowl, combine the fresh blackberries and sugar. Stir well to coat the berries with sugar. Cover the bowl and let it sit at room temperature for 24-48 hours, stirring occasionally. This process, known as maceration, will draw out the juices from the blackberries.

Combine with Vinegar: After the maceration period, strain the blackberry mixture through a fine mesh sieve into a clean bowl, pressing the berries to extract as much juice as possible. Discard the solids. Add the apple cider vinegar to the blackberry juice and stir to combine.

Store the Shrub: Transfer the shrub mixture to a clean jar or bottle and store it in the refrigerator. It will keep for several weeks, and the flavors will continue to develop over time.

Serve the Shrub Juice: To serve, fill a glass with ice cubes. Add 1-2 tablespoons of the blackberry shrub mixture, and top with sparkling water or club soda. Stir gently to combine. Garnish with fresh mint leaves if desired.