Place the potatoes in a medium saucepan with generously salted water. Place over heat and bring to a boil. Boil gently until tender, about 15 minutes. Drain and let come to room temperature while you make the dressing.
In a medium mixing bowl, whisk together the sour cream, mayonnaise, mustard, and vinegar. Give it a sample and add salt to taste. Add the potatoes and stir to coat. Add the radishes, cornichons, dill, and chives and stir gently one last time.
Transfer to a serving dish and garnish with chive blossoms, if using. Serve at room temperature.