Cut the limes in half and squeeze them using either an electric juicer or a hand-held juicer.
Add the fresh blackberries and mint to a large pitcher. Using a muddler, crush the leaves and fruit together until they release their juices and aromas.
Add the lime juice and agave syrup to the pitcher and give it a good stir to combine it with fruit and mint.
Add the sparkling water. Stir to combine.
Refrigerate the mixture for at least 15 minutes for the flavors to allow the flavors to meld and intensify.
Serving Instructions
Fill two glasses with ice cubes and pour the beverage mixture into both.
Garnish the glasses with lime slices, a few extra blackberries and a sprig of mint.
Notes
For a festive touch, dip the rims of the glasses in lime juice and then in a shallow dish of sugar before adding the beverage.