Place the beans in a large saucepan, and cover with an inch of water. Place over high heat, and bring to a boil. When the beans have come to a boil, drain, and return them to the same pot. Cover the beans with 2 inches of water, and stir in 1 tablespoon of garlic, the tomato, cumin, and chili powder. Bring to a boil over high heat, then reduce heat to low, and simmer until the beans are very soft, about 3 hours and 45 minutes, adding water as needed.
Once the beans have cooked, mash them with the remaining tablespoon of garlic, the oil, and salt to taste; use additional water as needed to achieve desired consistency. Place over low heat for 30 minutes, stirring occasionally. Serve.