Bring a pot of salted water to a boil and cook the potatoes until tender, about 10 minutes. Set aside. When cool, transfer to a large bowl and combine with black beans.
In a blender combine the scallions, sour cream and lemon juice. Pulse a few times to mix, then puree until smooth. Add salt, pepper and mayonnaise. Blend just to combine. Garnish with chopped chives.
Drizzle vinaigrette over salad, then toss until potatoes and beans are well coated.
c.1997, M.S. Milliken & S. Feniger, all rights reserved