Season chicken breast on both sides with salt and pepper. Brown in a skillet with vegetable oil for about 3 minutes per side, if desired.
Add chicken to the slow cooker. Pour in diced tomatoes, enchilada sauce, chicken broth, and undrained black beans.
Add thawed corn, onion, garlic, chili powder, cumin, and cilantro. Stir to combine.
Cover and cook on high for 2 hours, until chicken is cooked through and shreds easily.
Remove chicken and shred with two forks, then return it to the soup.
Taste for salt and pepper and serve with toppings.
Notes
Browning the chicken first adds flavor, but the soup still works if you skip that step. Do not drain the black beans because their liquid helps thicken the broth. Thaw frozen corn before adding so it does not cool the slow cooker. Use two cans of enchilada sauce for a fuller broth flavor.