Add the blackberries, water, and white sugar to a small saucepan. Turn your stove to medium-high until mixture begins to boil.
Once the mixture begins to boil, set your timer for five minutes and allow to cook and thicken. Be sure to stir the mixture during this time.
Remove from heat, and allow the mixture to cool for approximately 30 minutes.
Using a mesh strainer, separate the liquid from the seeds. Once separated, add the liquid simple syrup to a squeeze bottle, and discard the seeds.
Add the orange juice and mint leaves to the bottom of a cocktail shaker. Using a cocktail muddler or the back of a wooden spoon, muddle the orange juice and mint together until you can begin to smell the mint (approximately 30 seconds).
Add you simple syrup, vodka, and a handful of ice (about 8 cubes) to your cocktail shaker.
Shake the mixture until combined (approximately 30 seconds).
Pour mixture into a cocktail serving glass. Add your Fresca soda.
Top with additional ice cubes. Garnish with extra blackberries, mint, and an orange slice.
Be sure to add the Fresca soda to the glass, not the cocktail shaker. The Fresca soda will pop off the lid of the cocktail shaker if you add it with the rest of the mixture.