Add ingredients to a small bowl then stir to combine and set aside.
For the Soup:
Heat a large soup pot or Dutch oven over medium-high heat then add ground beef, season with salt and pepper, and brown, breaking it up as it cooks. Turn off heat then drain ground beef and add back into the pot along with the diced tomatoes and green chilies, black beans, taco seasoning mixture, and 1-1/2 cups milk. Stir to combine then set aside.
Heat a small saucepan over medium heat then add butter. Once melted, sprinkle in flour then whisk while cooking for one minute. Slowly stream in remaining 1-1/2 cups milk while whisking to create a smooth sauce. Season with salt and pepper then switch to a spatula and cook while stirring until the mixture has thickened, 3-4 minutes, being careful to not let it boil. Turn off heat then add half the shredded cheese and stir until smooth. Add remaining cheese then stir until smooth.
Add cheese sauce to the soup pot then stir to combine. Turn heat to medium-high then bring soup to a simmer, stirring often. Turn heat down to medium-low then simmer for 10 minutes, stirring often. Scoop into bowls then serve with toppings.
Notes
Mix up a a large batch of taco seasoning to keep on hand!