Whisk together vinegar, garlic, 1 tablespoon black pepper, rosemary, oregano, and kosher salt in a bowl along with the olive oil. Place the flat iron steak into a resealable plastic bag, and pour the marinade overtop. Squeeze the excess air from the bag and seal. Marinate in the refrigerator 30 minutes.
Preheat an outdoor grill for medium-high heat and lightly oil grate. Remove steak from the marinade, shake off excess, and discard the remaining marinade. Allow the steak to stand at room temperature for 15 minutes as the grill warms.
Mash together the butter, blue cheese, chives, and 1/8 teaspoon of black pepper; set aside. Cook the steak on the preheated grill to desired degree of doneness, about 5 minutes per side for medium-rare. Allow the steak to rest in a warm place for 10 minutes before slicing thinly across the grain. Serve with a dollop of blue cheese-chive butter.
