Combine the lemon zest and sugar in a large saucepan. Rub together with your fingers for 30 seconds to release the essential oils. Add the blueberries, chopped rhubarb, and water.
Stir to combine then place over medium high heat. Bring to a boil then simmer on Medium Low for 15 to 20 minutes, until fruit has become very tender and juicy. Use a fine mesh sieve and the back of a spoon to strain the fruit. For easier straining, let cool to room temperature then blend and strain.
Remove from heat to stir in the lemon juice.
To serve, add 1/4 cup concentrate into a glass of water and fill with about 1/2 cup to 3/4 cups water. The amount of water added will depend on how sweet you like your lemonade. Add ice to enjoy cold!