Combine blackberries and sugar in a large container.

Crush the blackberries and sugar mixture with a wooden spoon.

Stir in chopped fresh mint and baking soda (if using).

Boil 4 cups of water.

Pour the boiling water over the tea bags in a separate container.

Cover the container with the tea bags and let steep for 5 minutes.

Discard the tea bags.

Pour the hot tea over the blackberry mixture.

Let the mixture stand at room temperature for 1 hour.

Pour the tea through a wire-mesh strainer into a large pitcher, discarding solids.

Add 2 1/2 cups of cold water to the pitcher.

Stir until the sugar dissolves completely.

Cover the pitcher and chill in the refrigerator for at least 1 hour.

Garnish with fresh blackberries before serving, if desired.