Heat 1 tablespoon of the oil in a small skillet. Add the garlic, black beans and ginger and cook over moderate heat, stirring, until very fragrant, 2 to 3 minutes. Remove from the heat and whisk in the vinegar and mustard. Transfer to a bowl and whisk in the remaining 1/2 cup of oil. Stir in the scallions and season with salt and pepper. Gently fold in the potatoes. Serve warm or at room temperature.