In a medium saucepan over medium-high heat, combine sugar with 1 cup of cold water. Heat, stirring frequently until sugar dissolves. Cook for 3 minutes, before removing from heat and allowing to cool, about 5 minutes.

In a blender, combine 1 c. simple syrup, leaves from mint sprigs, and blackberries. Puree until smooth.

Using a fine mesh sieve, strain mixture into a bowl or glass, pressing down with a rubber spatula to get as much of the syrup as possible, about 1 cup. Refrigerate until cold - at least 1 hour, but as long as 3 days.

Next, transfer blackberry syrup to a large glass serving pitcher. Add bitters, tequila, and 1 1/2 c. cold water to the pitcher and stir. If serving immediately, proceed to the next step. Otherwise, the cocktail can be returned to the fridge for up to another day.

Add lime juice to pitcher and stir to combine. If serving immediately, proceed to the next step. Otherwise, the cocktail can be returned to the fridge for up to 2 more hours.

Serve: Stir cocktail mixture before pouring about 5 oz. of mixture into a glass filled halfway with ice. Top with 3-4 oz. sparkling wine. Garnish with additional springs of mint or slices of lime, if desired, and serve immediately.