In large zip-top bag, combine all ingredients, except butter and fresh blackberries. Squeeze out air and seal bag. Massage bag until well combined and marinade mix is dissolved. Marinate in refrigerator 6 to 8 hours. Remove filets from refrigerator 30 minutes before cooking.
Melt butter in heavy-bottomed, or cast iron, skillet over medium-high heat.
Remove steaks from marinade and sear for 3 to 5 minutes per side.
Serve hot, topped with pan juices and fresh blackberry garnish.