To prepare panna cotta, sprinkle gelatin over water in a small bowl; let stand 10 minutes. Bring milk, powdered sugar, and vanilla bean to a boil in a medium saucepan over medium-high heat. Remove pan from heat; remove vanilla bean with a slotted spoon. Scrape seeds from vanilla bean. Stir seeds into milk mixture; discard bean.

Add gelatin mixture to milk mixture, stirring with a whisk until gelatin dissolves. Add sour cream and cardamom; stir until well combined. Divide mixture evenly among 8 (6-ounce) custard cups. Cover and refrigerate for 8 hours or overnight.

To prepare compote, place 1 cup blackberries, wine, and granulated sugar in a food processor, and process until smooth. Strain the blackberry mixture through a fine sieve into a medium saucepan, and discard solids. Bring mixture to a boil over medium-high heat, and cook for 1 minute. Remove from heat; add remaining 2 cups blackberries. Cool completely.

Loosen edges of custards with a knife or rubber spatula. Place a dessert plate, upside down, on top of each cup; invert onto plates. Serve with compote; top with mint sprigs.