Place the 4 boneless skinless chicken breasts in the bottom of the slow cooker in a single layer.
Sprinkle 2 tablespoons taco seasoning evenly over the chicken.
Add the chopped green bell pepper, drained corn, rinsed and drained black beans, and 1½ to 2 cups salsa on top of the chicken.
Cover and cook on HIGH for 3½ to 4 hours (about 210 minutes) or until the chicken is cooked through and reaches a safe internal temperature.
Remove the chicken and shred it with two forks, then return the shredded chicken to the slow cooker and stir to combine with the juices and vegetables.
Serve the shredded chicken over cooked rice and top with chopped cilantro, green onions, sour cream, shredded cheese, and diced tomatoes as desired.