Heat the blackberries and grape juice until boiling. Lower heat and let simmer for 5 minutes, crushing the blackberries. Pour through mesh strainer, pressing the blackberries to get as much juice as possible. Chill the juice
To make the drink, fill a tumbler halfway with crushed ice. Add two shots (3 oz) of juice, 1 shot of vodka, and a splash of Triple-Sec. Top off the glass with tonic water and stir to combine.