First, preheat your oven to 350°F (175°C). While it's heating up, lightly grease your 9x9 baking dish with nonstick cooking spray. This will prevent the casserole from sticking and make cleanup much easier later on.
In your large mixing bowl, combine the cooked chicken, undrained Ro-Tel, uncooked instant rice, and 1 1/2 cups of the Colby Jack cheese. Add the cream of chicken soup, taco seasoning, milk, corn, and black beans if you're using them. Stir everything together until it's well combined.
Pour the mixture into your prepared baking dish and spread it out evenly. Sprinkle the remaining 1/2 cup of cheese over the top. Cover the dish tightly with aluminum foil and bake for 30 minutes, or until it's hot and bubbly.
Once it's done baking, carefully remove the foil. Let the casserole sit for about 5 minutes before serving. This allows it to set up slightly, making it easier to scoop and serve.