Whisk olive oil, vinegar, garlic, 1 tablespoon black pepper, rosemary, oregano, and kosher salt together in a bowl and pour into a resealable plastic bag. Add steak, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 30 minutes.

Preheat an outdoor grill for medium-high heat and lightly oil grate. Remove steak from the marinade, shake off excess, and discard remaining marinade. Allow steak to stand at room temperature as the grill warms.

Meanwhile, mash butter, blue cheese, chives, and 1/8 teaspoon of black pepper together until combined; set aside.

Cook steaks on the preheated grill until firm and reddish-pink and juicy in the center, about 5 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C). Allow 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.