Season with salt and pepper. Pour Blackberry Chipotle Sauce over steak. Turn to coat completely. (This can also be done in a large ziplock bag) Chill the marinade for at least an hour or up to 24 hours.
Prepare your grill to high heat. Remove steak from marinade and shake off excess marinade.
Place steak on the grill to sear for a minute or two on each side.
Reduce heat to Medium (set dials so outside is set to medium and center is off) brush with reserved blackberry chipotle sauce if desired, continue to cook turning once to your liking. For medium rare, remove steak when it reaches 125°F and allow to rest for 5-10 minutes.
Slice flank steak across the grain at a diagonal so the slices are wide but thin.