How to Make It
In a nonreactive saucepan, bring the currants and 1 1/2 cups of water to a boil over high heat, then reduce the heat and simmer for about 20 minutes. Line a strainer with a double layer of moistened cheesecloth and set over a nonreactive saucepan. Pour the currants into the strainer and let drip for 1 hour. Squeeze the cheesecloth gently to extract excess juice. You should have 2 1/2 cups. If you don't add a few tablespoons of water to make up the difference.
Place the currant juice over moderately low heat to warm slightly. Stir in 6 tablespoons of the sugar until dissolved. In a small saucepan, sprinkle the gelatin over 1/4 cup of cold water and let stand 5 minutes to soften. Warm over low heat, stirring, until dissolved. Stir the gelatin into the currant juice. Pour into a bowl, cover and refrigerate until set.
In a large bowl, crush about 1/4 cup of the blackberries; stir in the remaining whole berries. Sprinkle the remaining 3 tablespoons sugar on top and toss. Let stand until the berries exude some of their juice, about 5 minutes. Stir in the kirsch.
To serve, scramble the chilled gelée with a fork (the mixture will be very loose). Spoon some of the gelée into the bottoms of dessert glasses or wineglasses. Spoon the berries and their juice on top, garnish with a mint sprig and serve.