Whisk mayonnaise, ½ cup blue cheese, yogurt, buttermilk, and vinegar in medium bowl until almost smooth. Mix in remaining blue cheese. Season dressing generously with cracked black pepper. (Can be made 1 week ahead. Cover and refrigerate.)
Arrange lettuce wedges on 8 plates. Spoon dressing over lettuce. Garnish with tomatoes and crispy bacon pieces if desired. Sprinkle with onions and additional black pepper and serve.