In a pitcher, combine the blackberry purée, lemon juice, elderflower liqueur, and gin. Stir well to combine.
Cover and refrigerate until thoroughly chilled.
When ready to serve, divide the mixture evenly between four glasses filled with ice.
Top each glass with 3 oz tonic water and, if desired, 1 oz sparkling water or Prosecco.
Gently stir to combine.
Garnish with freshly halved blackberries threaded onto cocktail picks.
For a Halloween presentation: Place the serving glass inside a larger dish containing water and a small piece of dry ice to create a fog effect without allowing dry ice to touch the cocktail. Use protective gloves or tongs when handling it.