Add blackberries, sugar, ice, gin, and mint into a cocktail shaker in that order, and using a muddler, muddle until berries are pulverized.
Add the remaining ice into 2 12 oz glasses (tall glass if possible). Remember to divide the ice equally.
Strain the blackberry mixture over the top of the ice. Divide evenly into both glasses.
Squeeze lemon juice from half a lemon over the top of each cocktail.
Carefully top with club soda to fill the glass.
Garnish with a lemon wedge or lemon slice and a sprig of mint.