Combine gin, lemon juice, and simple syrup in a cocktail shaker with ice.
Put the lid on the shaker, and shake vigorously for thirty seconds.
Fill a rocks glass with crushed ice, then strain the contents of the shaker into the glass.
Drizzle the blackberry liqueur on top of the cocktail.
Garnish with a lemon wheel and blackberry, and serve immediately.
Notes
2 different ways to make simple syrup:
Combine equal parts sugar and hot water in a jar with lid. Seal shut, and shake until the sugar is dissolved. Cool until ready to use.
Combine equal parts sugar and water in a small sauce pan over low heat. Stir until sugar is dissolved. Cool until ready to use.
You can start by making a small batch of simple syrup - 1/2 cup each of sugar and water. But as long as you have equal parts of both, a 1:1 ratio simple syrup can be made in small or large batches.