In a large skillet, cook and crumble beef over medium-high heat until no longer pink, 4-6 minutes. Stir in tomatoes; bring to a boil. Reduce heat; simmer, uncovered, until liquid is almost evaporated, 6-8 minutes. Stir in black beans; heat through.
To serve, spread refried beans over tostada shells. Top with beef mixture; add toppings as desired.