In a medium sauce pot, combine blueberries, blackberries, lemon juice, vanilla extract, and sugar. Set heat to medium, mix and cover the pot with a lid.
Bring mixture to simmer and lower the heat to medium-low. Let the mixture cook, stirring often, for about 15 minutes. (Be careful so that the mixture doesn't over-run. Leave a little crack in the lid so the heat can escape.)
Take off heat.
Bring 3 quarts of water to boil and turn it off.
Steep 6 green tea bags until strong tea is reached. You want the tea to be a little stronger that a usual hot cup of tea because it will be served with ice.
After desired strength is reached, take out the tea bags.
Stir if blueberry-blackberry mixture into the tea. Let it cool to room temperature.
Transfer the tea to a pitcher with a lid and refrigerate until cold and ready to serve.