Place the chicken breasts in the slow cooker in a single layer; they can be frozen or thawed.
Pour the drained black beans and drained corn over the chicken, then add the salsa. Sprinkle the taco seasoning over the top if using.
Cover and cook on Low for 4 to 5 hours, or until the chicken reaches a safe internal temperature and is tender enough to shred.
Remove the chicken to a cutting board and shred with two forks or tongs; return the shredded chicken to the slow cooker.
Add the softened cream cheese on top of the shredded chicken, cover, and let sit for about 30 minutes to soften, then stir until the cream cheese is fully incorporated into the sauce.
Stir well to combine and serve the fiesta chicken over rice or in warm flour tortillas.
