Lay out flank steak on cutting board or parchment paper. Score the steak with a knife to allow marinade to seep into the steak. Set aside.
Put marinade ingredients in a small bowl. Stir until well combined. Transfer meat and marinade to a double-lined freezer bag. Be sure all parts of the meat get covered in the marinade by squishing it all around. Let it rest for at least 30 minutes or overnight if possible.
Turn on your oven's broiler.
On a roasting rack, cook the meat under the broiler for 6 minutes per side for medium-rare cooked steak. Take steak out of the oven once both sides have cooked and let rest for 10 minutes.
While steak is resting, combine all ingredients in a small sauce pan. Over medium heat, boil blueberry mixture until blueberries break down and mixture is reduced (about 6-8 minutes).
Pour blueberry balsamic mixture over cut steak and enjoy!
*If you're trying to keep it refined sugar free, feel free to substitute maple syrup (1 Tbsp) instead of brown sugar!