In a small bowl, whisk together the vinegar, oil, sugar, garlic powder, lime, salt and pepper. Set aside.
Empty can of black beans into a colander and allow to drain well in the sink but do not rinse with water.
Set barbecue grill to medium heat. Husk corn and place on grill. Grill, covered, for about 12-15 minutes. Rotate corn once or twice while grilling. Remove from grill and allow to cool for a few minutes.
Run a sharp knife down the sides of the corn cob to remove the grilled kernels. In a medium serving bowl combine the drained black beans, cherry tomatoes, red bell pepper, green onions, cilantro, and corn. Toss with dressing. Do not add avocado, that will come later. Cover and chill while preparing steak or for up to 4 hours.
Place steak on a large platter or baking sheet. Sprinkle generously with olive oil, salt and pepper, on both sides. Place on grill set to medium heat and grill for about 7 to 8 minutes on each side for medium rare to medium, depending on thickness. Cook a few minutes longer for well done. Remove from grill and allow steaks to sit for 5 or 10 minutes before serving.
Remove salsa from refrigerator and add chopped avocado. Toss lightly to combine.
Garnish each piece of steak with a scoop of the grilled corn and black bean salsa and serve.