Roast the poblanos over a gas flame or under the broiler until charred all over. Transfer to a small bowl, cover with plastic wrap and let stand until cool.
Peel the blackened skin from the poblanos and discard the cores, seeds and ribs. Coarsely chop the poblanos and transfer to a blender or food processor. Add the scallions, garlic, cilantro and lime juice and blend until roughly chopped. Add the buttermilk, olive oil and sugar and puree until smooth. Season with salt and pepper.
The dressing can be refrigerated, covered, for up to 1 day.
One Tablespoon Calories 27 kcal, Protein .4 gm, Carbohydrate 1.2 gm, Cholesterol .4 mg, Total Fat 2.4 gm, Saturated Fat .4 gm.