Add water and salt to large pot and bring to a boil. Add potatoes and cover, cooking until tender, about 25 to 30 minutes. Drain and slice while warm. Place in a large bowl; set aside.
In a small bowl, mix vinegar, mustard, salt, garlic powder, marjoram and pepper. Using a whisk or fork, gradually beat in oil until well combined. Pour oil mixture over warm potatoes, stirring gently to coat potato slices. Cover and refrigerate for at least 2 hours (or as long as overnight, if you wish).
Remove from refrigerator and lightly mix in radishes, cucumber, onion, carrot and eggs. In a small bowl, mix mayonnaise and sour cream until smooth; add blue cheese cheese and mix lightly to incorporate. Fold mayonnaise mixture into potatoes. Cover and refrigerate for 1 to 3 hours. Serve cold.