Cut potatoes in 1" dice without peeling, and place in large pot or dutch oven. Cover with cold water by 1" and salt generously. Heat, covered, on high until boiling, then remove lid and allow to cook completely through, 10 to 12 minutes. In order to make sure the potatoes are cooked perfectly, I take one out of the pot, cut it in half with a fork, and taste it for "done-ness": you definitely don't want the potatoes overcooked, but crunchy spuds do not make for a good salad either.
Meanwhile, in large bowl, combine mayonnaise, sour cream, blue cheese, celery, scallions, about 3/4 teaspoon of salt and 1/2 teaspoon of ground black pepper and mix well.
When potatoes are cooked through, drain well in colander, and return to pot. Sprinkle hot potatoes with vinegar and toss to coat. Allow to cool for about ten minutes.
Add cooled potatoes to mayonnaise mixture and toss gently, using a folding motion, to coat well. Cover and refrigerate for at least 3 hours, or overnight. Mix potato salad again before serving.