In a medium saucepan, melt 2 tablespoons sugar in 2 tablespoons water over medium-low heat; let cool. Transfer to a pitcher, add 1 cup blackberries and 1/4 cup fresh sage leaves and muddle. Stir in 2 cups citron vodka, 1/4 cup amaretto, 1/4 cup cassis and 1/4 cup orange juice. Fill 8 highball glasses with ice. Pour in the mixture and top with seltzer. Garnish with more sage leaves and blackberries.