Using muddler or handle of wooden spoon, muddle 10 blackberries in cocktail shaker. Add sloe gin, mezcal, lemon juice, and cane syrup. Shake without ice until well blended, about 10 times. Divide mixture between 2 large rocks glasses. Fill each glass with enough crushed ice to reach top of glass. Garnish each with additional blackberries and serve.
10 fresh blackberries
1/4 cup Plymouth sloe gin
1/4 cup mezcal (such as Sombra or Del Maguey Vida)
3 tablespoons fresh lemon juice
2 tablespoons pure cane syrup (such as Steen’s)
3 cups (about) crushed ice
Additional fresh blackberries (for garnish)