Fill a shaker with ice. Add the gin, lemon juice, and simple syrup. Shake well for about 10–12 seconds until cold. Strain into a tall glass (Collins or highball) filled with fresh ice. Top with soda water — about 2–3 oz, or to taste. Garnish with a lemon wheel or lemon twist. To make the blackberry mask, pour the ¼ oz blackberry syrup gently over the top. It will drift downward and create a dark “mask” streak effect.