Prepare the blueberry puree: In a blender, puree 1 cup of fresh or thawed frozen wild blueberries until smooth. Strain the puree through a fine-mesh sieve to remove the skins and seeds. Set aside 1 oz of the strained puree for the cocktail, and save the rest for other uses or future cocktails.

In a mixing glass or shaker, gently muddle the mint leaves with the maple syrup to release the mint's flavor. Be careful not to tear the leaves, as this can cause bitterness.

Add the bourbon, lemon juice, and blueberry puree to the mixing glass. Fill the shaker with ice and shake well until chilled and combined.

Fill a rocks or julep glass with crushed ice, and strain the cocktail mixture over the ice.

Garnish with fresh wild blueberries and a mint sprig.