Muddle the blackberries in the bottom of a cocktail shaker.
Add in the gin, simple syrup and ice. If you don't care about seeing the color change in the glass, feel free to add the lime juice in during this step.
Add the lime juice (unless you already shook it up) and fresh ice to an old fashioned cocktail glass.
Strain the cocktail onto the ice. If you didn't shake with the lime juice, give it a few stirs with a cocktail spoon.
Place two blackberries on a cocktail pick for garnish, and serve.