Combine lemon juice, oil, onion, and garlic in large plastic zip lock bag; mix well.
Place steaks in bag and seal. Refrigerate for 6 to 24 hours, turning bag occasionally.
When ready to cook, rub pepper around the edges of each steak. Preheat grill.
In small saucepan add jam, vinegar, and onion powder. Cook over medium heat until jam is melted, stirring constantly. Remove from heat.
Place steaks on gas grill over medium heat, or charcoal grill 4 to 6 inches from medium-hot coals. Cook 8 to 12 minutes or till desired doneness turning once halfway through cooking.
To serve, spread steaks with sauce and top with fresh blackberries. steaks can also be cooked inside broiler oven by placing on broiler pan 4 to 6 inches from heat and broiling 7 to 10 minutes or until done.