Cook the pasta according to package directions until al dente, drain in a colander, and set aside.
Preheat the broiler to high and position a rack so the skillet will be a few inches from the heat.
In a very large oven-safe skillet over medium-high heat, melt the butter.
Add the flour to the melted butter and whisk constantly until the mixture turns light brown and bubbles for about 2 minutes to cook off the raw flour taste (make a roux).
Slowly whisk in the milk, adding a little at a time and smoothing any lumps until the sauce is smooth.
Whisk in the taco seasoning and black pepper until evenly combined.
Add 3 cups (about 12 ounces) of the grated cheddar and whisk until melted and the sauce is smooth.
Stir the cooked pasta, drained black beans, and sliced green onions into the cheese sauce until everything is evenly coated.
Sprinkle the remaining 1 cup grated cheddar evenly over the top and place the skillet under the broiler 1 to 2 minutes until the top is melted and bubbling; watch closely to avoid burning.
Remove from the oven and evenly sprinkle the diced tomato, reserved 2 tablespoons green onions, and minced cilantro over the top; serve immediately.