Juice the lemons until you have two cups of fresh juice.

In a blender, combine the blueberries and blackberries and blend until smooth.

Strain the berry puree into a bowl or pitcher to remove the seeds.

In a large pitcher, combine the lemon juice, strained berry puree, and granulated sugar, and stir until the sugar is fully dissolved.

Pour in the six cups of water to dilute the mixture, adjusting to taste.

Refrigerate the lemonade for at least an hour to chill.

Fill glasses with ice and pour the lemonade over the ice.

Garnish with lemon slices or extra berries if desired.