1. Combine 3 cups of the beans and 1 cup of chicken broth in a blender or food processor and puree until smooth.
2. In a large saucepan, heat the oil over moderate heat. Add the onion and 1 teaspoon of the salt and cook, stirring frequently, until the onion is soft, about 5 minutes. Increase the heat, add the sherry, and boil until reduced to approximately 1/4 cup, about 3 minutes.
3. Add the bean puree, 1/2 teaspoon of the pepper, and the remaining 3 cups of chicken broth and 3 cups of beans. Simmer until hot, about 5 minutes.
4. Meanwhile, in a small bowl, combine the avocados, cilantro, radishes, lime juice, and the remaining 1/2 teaspoon each of salt and pepper. Mix gently. Spoon the soup into bowls and top with the salsa.
Wine Recommendation: Don't forget about sherries when considering wines for the table. They work particularly well with soups, especially when sherry is an ingredient already. With this black-bean soup, a dry amontillado is a natural.
Notes: If the radishes aren't perfectly crisp, soak them in a bowl of ice water. A few minutes of shock may be just enough to revive them.