Combine blackening seasoning in a small bowl. Rub on both sides of steak. Cook steak on grill 4 min on each side or until it has reached desired doneness. Remove from grill and let rest 5 minutes before slicing. While steak is cooking, arrange arugula evenly in each of 4 large bowls. Top evenly with cucumber, bell pepper, and onion. Slice steak into thin strips. Arrange strips over salads. Drizzle with balsamic vinaigrette. Top with cheese. Top with croutons, if desired.
Combine all ingredients in a blender. Blend until well combined. (If you don’t have a blender, you can shake them together in an airtight container or whisk them together in a bowl.) with cheese. Top with croutons, if desired.
Preheat oven to 375. Cut bread into cubes. Place in a bowl. Drizzle with oil and garlic powder. Toss well to combine. Arrange in a single layer on a baking sheet. Bake at 375 for 10 minutes. Serve immediately or store in an airtight container.