Begin by rinsing and draining the black beans. Mince the red bell pepper and dice the scallions.
In your slow cooker, combine the rinsed black beans, chicken broth, Rotel tomatoes, minced red bell pepper, and canned green chiles. Stir in the ground cumin, ancho chile powder, garlic powder, onion powder, and oregano. Mix everything well.
Place the boneless, skinless chicken breasts into the slow cooker, ensuring they are submerged in the liquid.
Cover the slow cooker and set it to low for 6-8 hours or high for 3-4 hours.
Once the cooking time is up, remove the chicken breasts from the slow cooker and shred them using two forks. Return the shredded chicken back into the pot.
Stir in half of the chopped cilantro and most of the diced scallions. Taste and adjust seasoning if necessary.
Ladle the soup into bowls and top with remaining cilantro, diced scallions, avocado slices, and a dollop of sour cream if desired. Serve with lime wedges on the side.