Add the blackberries, basil, honey, and water to a small saucepan set over medium heat. Simmer, stirring occasionally, until mixture is heated through and starts to thicken, approximately 4-5 minutes. Gently press on blackberries while heating to release juices.
Remove from heat and strain mixture through a fine-mesh strainer to remove solids. Reserve the remaining liquid and set aside to cool.
If using, divide the Elderflower liqueur among four champagne flutes, along with a drizzle of the blackberry simple syrup. Finish by topping off each glass with some chilled Prosecco.
Garnish each glass with 2-3 fresh blackberries and a sprig of fresh basil, if desired, before serving. Enjoy!