The day before you're going to make the stew, sort and rinse through the beans, place them in a large bowl and cover them with about 2 inches of water. Leave them overnight to soak. The next day, drain the beans and set them aside.
Heat the oil in a large heavy pot, then add the salt pork, onions, garlic, cumin, and dried chiles. Stir to combine and toast the spices a little bit, then add the beans, sugar, and bay leaf, along with 1 quart (4 cups) water and salt and pepper. Bring to a boil, then turn down the heat and simmer until the beans are soft, about 1 1/2 hours.
Puree 1/2 cup of the beans and any large pieces of chile until smooth, then return them to the pot. Simmer for another 20 minutes, or until the mixture thickens slightly. Taste for salt and pepper. Remove and discard the bay leaf. The stew texture should be soupy, but punctuated with beans.
To serve: Ladle some beans and their liquid over rice, and top with shredded cheese, scallions, cilantro, and sour cream.