In a large heavy saucepan, heat the olive oil over moderate heat for 1 minute; add the onion and garlic and cook, uncovered, for 5 minutes or until the onion is soft.
Stir in the oregano, thyme, cumin, and cayenne pepper, and cook, stirring, 1 minute longer.
Meanwhile, place half of the black beans in an electric blender or food processor and puree by whirling for 30 seconds.
Add the bean puree, the remaining beans, and the chicken broth to the saucepan, reduce the heat to low, and cook, uncovered, for 15 minutes.
Ladle the soup into bowls and garnish, if you like, with the coriander.