Recipe via a Dash of Texas Blog:

"We would make awful bartenders. We learned early on in our relationship that mixology (is that a pretentious word?) is not one of our strengths. The evening begins innocently enough; the seed of an idea for a drink is planted, or we’re influenced by an online recipe. But it usually ends in disaster – our end product never tastes quite right, and we spend a small fortune to buy all of the ingredients we didn’t have before. As a result, our top pantry shelf looks like we’re raging alcoholics because it’s stocked with a dozen bottles of stuff we don’t know what to do with."

2 cups whole blackberries (frozen is fine, just thaw and drain)
1/2 cup granulated white sugar
Juice of 2 limes
2 cups gin
Topo Chico (or any other sparkling water)

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.