Bring the frozen blueberries and ½ cup fresh lemon juice to a boil in a large saucepan over medium heat. Cook, stirring occasionally, 5 minutes.
Remove from heat, and pour through a fine wire-mesh strainer into a bowl, using back of a spoon to squeeze out juice. Discard solids. Wipe saucepan clean.
Bring 1 litre of water to a boil in same saucepan; add 3 tea bags and leave to brew for 5 minutes.
Remove and discard tea bags. Stir in the Stevia and blueberry juice mixture.
Pour into a pitcher; cover and chill 1 hour. Serve over ice.
