Blackberry Lemonade

4 cups water, divided
1 cup sugar
3/4 cup lemon juice
1 tbsp grated lemon zest
1 cup blackberries

Bring 2 cups water and the sugar to a boil in a medium saucepan. Boil for 2 minutes, stirring occasionally.
Remove from heat and stir in the lemon juice, lemon zest, and remaining 2 cups water.
In a blender, process the blackberries with 1 cup of the lemon mixture until smooth. You can strain the seeds if desired, or leave them in. 
Add the blackberry mixture back into the saucepan with the remaining lemon mixture and stir together. Pour the drink into a pitcher and chill before serving.