In a large bowl, whisk together the oil, yogurt, Worcestershire, garlic, and salt and pepper, to taste. Stir in the blue cheese and chives, then add a little buttermilk to reach your preferred consistency. Cover and refrigerate at least 1 hour before serving. Tip: It’s a good idea to keep the dressing on the thick side, as the juice from the tomatoes will thin it slightly. You can thin more for use as a salad dressing, if you like.