Combine all of the rub ingredients in a pie plate.
Rub chicken breasts on both sides with 2 teaspoons of olive oil. Dredge chicken breasts on both sides with the blackening spice. Discard any remaining rub.
In a 4-5 quart slow cooker, layer half of the tomatoes, peppers, and onions. Top with chicken breasts. Top with remaining tomatoes, peppers, and onions.
Cover and cook on high heat for 3-4 hours, or on low for 4-6 hours, or until a meat thermometer inserted in the chicken registers at least 165 degrees.
Serve on wheat tortillas with cilantro, lime wedges, and avocado.