You Can Omit The Vodka, for an all ages drink.
Place 1 cup water and 3 cups blackberries in a blender; process until smooth. Press blackberry puree through a sieve into a large pitcher; discard seeds.
Add remaining 5 cups water, sugar, limeade and vodka to pitcher; stir until sugar dissolves.
Place 1 lime slice and a few blackberries, if desired, into each of 8 glasses; pour about 1 cup limeade over each serving.