In a cocktail shaker or a jug, muddle the fresh blackberries and mint leaves together until they release their juices and aromas.

Cut one lime in half and squeeze the juice, make sure to remove any seeds that may fall in.

Add the agave syrup and give it a good stir to combine all the ingredients.

Fill a glass with ice cubes and pour the blackberry and mint mixture over the ice.

Top the glass with sparkling water, leaving some space at the top for garnish.

Stir gently to mix the flavors together.

Drop a few extra blackberries into the glass for added flavor and visual appeal.

Give the mocktail a final stir and serve immediately.

Optionally, you can garnish the glass with a sprig of fresh mint or a lime wedge on the rim.