Place the beans, chicken broth, water, onion, garlic, cumin, and oregano in a blender or a food processor. Process the ingredients for 1 minute; turn off the machine and wait until the blades stop, then scrape down the sides of the container. Process the mixture for another minute, until smooth. Transfer the mixture to a medium-sized saucepan and bring to a boil. Reduce the heat to low and simmer, uncovered, for 15 minutes, stirring frequently.

Place in serving bowls and garnish with the chopped bell pepper and sour cream, if desired.