Add blackberries, rosemary, lemon juice and honey to a cocktail shaker or jar.
Muddle all of the ingredients together, squishing everything well in order to release the juices. Once everything is well-muddled, add the mezcal and orange bitters.
Strain the drink into an ice-filled glass. Top the glass with a little more ice, pour the seltzer water over, and gently stir to combine. Garnish with fresh rosemary. If you’re feeling bold, add a mezcal float to amp up the smoky flavor.