Place potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and let cool, about 15 minutes. Cut potatoes into chunks; transfer to a large bowl. Stir in hard-boiled eggs and green onions. Mix blue cheese dressing, sour cream, salt, and pepper together in a bowl. Pour over potato mixture; stir gently until blended. Chill until flavors combine, about 2 hours.