Blue Cheese Potato Salad

2 1/2 pounds potatoes, cooked and cubed
3/4 cup chopped scallion (green and white parts)
3/4 cup chopped celery
3/4 cup sour cream (regular or fat-free)
1/4 cup mayonnaise (regular or reduced-fat)
2 tablespoons minced fresh parsley
1 tablespoon white wine vinegar or cider vinegar
1 1/2 teaspoons salt
1/4 teaspoon pepper
1/4 teaspoon celery seed
1/2 cup crumbled blue cheese plus extra for top

In a large bowl toss together all the ingredients until very well mixed. Sprinkle the top with extra blue cheese. Cover and refrigerate for at least several hours before serving.