Combine water, sugar, and mint in a small saucepan over medium heat. Bring to a boil, stirring occasionally. Continue to boil for a 2-3 minutes until sugar is all dissolved.
Remove from heat. When slightly cooled, pour through a strainer to remove the mint leaves.
Note: Unless you like your iced tea really sweet, you probably won't be using all of the simple syrup. Use leftover syrup for other cool summer drinks.
Meanwhile, purée 2 cups of blueberries with one cup water in a food processor. Strain mixture through a fine mesh strainer so you have a thin(ish) juice.
Prepare the iced tea using 32 oz. (or more) of cold water and 4 (or more) cold brew tea bags. You can also make iced tea with regular tea bags if that's what you have. The cold brew type just speed things up a bit. Or buy a bottle of unsweetened iced tea.
Mix the blueberry purée, iced tea, and mint simple syrup, to taste, with plenty of ice. Stir, taste, add more simple syrup as needed, and enjoy!
Make ahead tip: The mint simple syrup can be made a week or more in advance. Store it in a clear glass jar in the fridge. Make the blueberry puree in the morning and refrigerate, as well as the iced tea. Mix it all together when you're ready to serve.
Substitute a bottle of pre-made unsweetened iced tea instead of brewing your own.
Nutrition information is based on using all of the simple syrup.