Combine fennel seed, 1/2 cup balsamic vinegar, 1-1/2 tablespoons olive oil, salt and pepper in small bowl; mix well. Place beef steak and marinade in food-safe plastic bag; turn steak to coat. Close bag securely; marinate in refrigerator 6 to 24 hours.
Remove steak from marinade; discard marinade. Place steak on grid over medium, ash-covered coals; grill steak, covered, 11 to 16 minutes (over medium heat on preheated gas grill, covered, 16 to 21 minutes) for medium rare (145°F) to medium (160°F) doneness, turning occasionally. Carve steak diagonally into thin slices.
Meanwhile cut ciabatta in half lengthwise; brush cut sides with remaining olive oil. Grill cut-side-down, during last 2 minutes of grilling beef
Place steak slices on bottom half of bread, top with arugula and blue cheese; drizzle with remaining balsamic vinegar. Close sandwich.