In a cocktail shaker or a jug, muddle the fresh blackberries and mint leaves. Press down until they release their juices and aromas.
Take one lime, cut it in half, and squeeze the juice right into your blackberry-mint mix.
Pour in that tablespoon of agave syrup.
Stir everything together to let the flavors combine.
Fill a glass with ice cubes. Now, carefully pour your blackberry and mint mixture over the ice.
Top the glass with sparkling water, leaving a bit of room for garnish.
Drop in a few more blackberries and a slice or two of lime. Stir gently to let everything swirl together.
For the final touch, you can pop a sprig of mint or a lime wedge on the rim.