Preheat the broiler. Put the poblanos on a baking sheet and broil, turning halfway through, until charred all over, 15 to 20 minutes. Let cool slightly, then remove the stems and seeds and peel off the charred skin. (It’s OK if some skin remains.)
Transfer the poblanos to a blender and add 1/2 cup water, the black beans, cilantro, scallions, lime juice, 2 teaspoons salt and the cumin. Puree until smooth. Transfer to a small bowl and serve with tortilla chips.