Recipe via a Dash of Texas Blog:
"We would make awful bartenders. We learned early on in our relationship that
2 cups whole blackberries (frozen is fine, just thaw and drain)
1/2 cup granulated white sugar
Juice of 2 limes
2 cups gin
1. In a blender, combine the blackberries and sugar. Blend until liquefied. Strain into a container through a fine mesh sieve, discarding any large pulp or seeds that remain.
2. At this point, you can combine all ingredients except the Topo Chico in a pitcher and pour over ice, topping with the sparkling water. If you're making individual drinks, portion out into a shaker, shake well, and pour over ice. Top with Topo Chico, a wedge of lime, and any leftover berries.
The ratios of gin to lime juice to blackberry puree is up to your individual taste, so add more or less of whichever ingredients you choose.