In a large saucepan, over medium heat, add the oil. When the oil is hot add the sausage and brown for 2 minutes. Add the onions and peppers. Season with salt and pepper. Saute for 3 to 4 minutes or until the vegetables are wilted. Stir in the garlic and black beans. Saute for 1 minute. Add the stock and bring the liquid to a boil, reduce the heat to medium-low and simmer until the beans are creamy and tender, about 2 1/2 hours, stirring occasionally. Stir in the butter and reseason with salt and pepper.