Begin by placing the boneless, skinless chicken thighs into the slow cooker. Season them generously with salt, pepper, ground cumin, and chili powder, ensuring they're well coated with the spices.
Next, layer the black beans, corn kernels, diced bell peppers, and diced onions over the seasoned chicken. Pour half of the enchilada sauce on top, and gently stir everything to distribute the ingredients evenly.
Cover the slow cooker with its lid, then set it to cook on low heat for 6-7 hours or on high heat for 3-4 hours, until the chicken is tender and can be easily shredded with a fork.
Once the cooking time is complete, carefully remove the chicken thighs from the slow cooker. Using two forks, shred the chicken into bite-sized pieces. Return the shredded chicken back into the pot, and stir well to mix it with the vegetables and sauce.
To prepare the tortillas, warm them gently in the microwave for about 15-20 seconds to make them pliable, which will make rolling easier.
Place a generous scoop of the chicken mixture into the center of each tortilla. Sprinkle a small handful of shredded cheese on top of the chicken, then roll the tortilla tightly to encase the filling.
Arrange the rolled enchiladas seam-side down in the slow cooker. Pour the remaining enchilada sauce over the top of the enchiladas and sprinkle the remaining shredded cheese generously on top.
Cover the slow cooker again and let it cook on low for an additional 30 minutes, allowing everything to heat through and the cheese to melt beautifully.
When ready to serve, garnish the enchiladas with freshly chopped cilantro. Add a dollop of sour cream on the side if desired.