Puree orange juice, zest, oil, shallot, vinegar salt, pepper, and Dijon in a blender or mini prep food processor. (Alternatively whisk in a bowl.)Reserve ¼ cup dressing and refrigerate. Drizzle the remaining dressing over the steak, onion and dates in a medium bowl and toss to coat.
Preheat grill to medium-high heat. Thread steak, onion and dates onto 8 skewers, alternating.
Oil grill rack by dragging an oil soaked paper towel over the grill grates (holding with tongs.) Grill skewers, rotating often until the meat is medium rare and the onions and dates are softened, about 8 to 14 minutes.
Divided spinach and arugula among four plates or pasta bowls. Top with the skewers. Drizzle with the reserved dressing (about 1 tablespoon per salad) and top with blue cheese.
