Make blueberry syrup: In a small saucepan, combine blueberries, sugar, and water. Bring to a simmer on medium heat and cook for about 10 minutes, stirring now and then, until the berries burst. Strain through a fine mesh strainer, pressing on the solids. Cool it down completely.
Prepare your glass: Toss mint leaves and fresh blueberries into the bottom. If you’re using powdered sugar, add it now.
Muddle gently: Press down with a muddler (or spoon) to release mint oils and blueberry juice. Go easy—too much muddling makes the mint bitter.
Add bourbon and syrup: Pour in the bourbon and blueberry syrup.
Add ice: Fill the glass with crushed ice. Stir gently for about 30 seconds to chill everything nicely.
Garnish: Top with a mound of more crushed ice, extra blueberries, and a mint sprig. Serve right away with a short straw