Bring 8 cups of water to a boil in a large saucepan over medium heat. Add blackberries, mint, and tea bags. Remove from heat and let stand 1 hour at room temperature. Strain the mixture through a sieve, into a pitcher and discard the tea bags and solids. Add sugar and lemon juice to the tea, stirring until sugar dissolves. Pour into glasses or cups and garnish each serving with a mint sprig.