Muddle* blackberries, mint leaves, juices and sugar in a tall glass until berries are mostly crushed and mint releases flavor. (If you don’t have a muddler, a wooden spoon or firm silicon spatula works well).
Add seltzer, vodka or rum, and ice to fill glass. Stir well. Garnish with a lime wedge and mint leaves.
*To muddle is a cocktail term meaning to gently crush. Click here to see a muddler. I had to look it up the first time I saw the term!