Place the chicken breasts in the bottom of the slow cooker. Add the black beans, salsa, cumin, chili powder, diced onion, and minced garlic on top.
Cover and cook on low for 6 to 8 hours until the chicken is very tender and shreds easily.
Use two forks to shred the chicken directly in the slow cooker. Stir everything together so the chicken absorbs the sauce.
Spoon the cooked rice into four bowls. Top each bowl with a portion of the shredded chicken and black bean mixture.
Add sliced avocado, fresh cilantro, and a squeeze of lime to each bowl before serving.