In a large skillet, brown beef until fully cooked. Drain off any excess grease.
Stir in tomatoes and bring to a boil. Reduce to simmer. Simmer uncovered for 6-8 minutes until most of the liquid is absorbed. Stir in black beans and heat through.
To serve, spread refried beans over tostada shells. Top with beef mixture along with toppings as desired. Serves 4.