Chop avocado flesh and mash in a bowl. Add tomatoes, onion, lime juice, garlic, cilantro, and salt. Cover with a lid or plastic wrap and refrigerate guacamole until ready to serve.
Preheat the oven to 375 degrees F (190 degrees C).
Heat a large skillet over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Add water and taco seasoning; stir until combined. Simmer until thickened, 3 to 4 minutes.
Spread refried beans into the bottom of a 9x13-inch baking pan. Top with black beans. Spread ground beef mixture on top of beans. Drizzle tomato sauce over the whole mixture.
Bake in the preheated oven for 15 minutes. Sprinkle Mexican cheese blend on top. Continue baking until cheese is melted, about 1 minute more.
Top each serving with guacamole, sour cream, and scallions.