Preheat grill. Toss potatoes with 1/4 cup olive oil and season with salt and pepper, to taste. Grill cut side down for 3 minutes. Turn over and continue grilling 2 to 3 minutes. Place in a large bowl.
Place medium skillet on the side burner or the grill and heat until almost smoking. Add the bacon and cook until golden brown and crisp. Remove to a plate lined with paper towels. Let cool and crumble.
Remove all but 2 tablespoons of the bacon fat from the pan. Add the onions and cook until soft, 5 to 6 minutes. Add 1/4 cup olive oil, vinegar, and sugar and cook until sugar is dissolved. Pour the onion mixture over the potatoes, add the crumbled bacon and parsley and toss to combine, season with salt and lots of black pepper. Spoon the salad onto a large platter and sprinkle with the crumbled blue cheese.