In a medium saucepan, heat another 2 tablespoons of the oil. Add the chipotle, cumin, chopped garlic and the remaining onion and cook over moderate heat, stirring occasionally, until the onion is softened, about 5 minutes. Add the beans with their liquid and 1/4 cup of water and cook over low heat for 15 minutes. Transfer the black bean mixture to the blender and puree until smooth. Season the sauce with salt.