In a medium-sized saucepan on medium-high heat, add the vegetable oil, onion, garlic and cumin. Sauté, stirring occasionally, until translucent and fragrant, about 5 minutes.
Add the bay leaves, rinsed black beans, Pasilla Oaxaca chiles and water.
Bring beans to a boil. Leave them uncovered and adjust the heat as necessary to maintain a gentle simmer. Cook beans for an hour and a half, or until tender.
Once the beans are tender, bring the heat up to medium-high and reduce the liquid until it's thick and gravy-like, about 5-10 minutes. Remove and discard bay leaves and Pasilla Oaxaca.
Add the salt, sour cream and lime juice and blend with an immersion blender until beans are smooth and creamy. Adjust salt to taste.
Serve warm with tortilla chips. Store any leftovers in the refrigerator.
Notes:
Garnish suggestions - Sprinkle queso fresco cheese, avocado, pickled peppers or your favorite salsa.