Place pork tenderloin into a slow cooker. Pour taco sauce over the pork, cover, and cook on High for 2 ½ to 3 hours.
Remove pork from the slow cooker and let rest for 5 to 10 minutes. Shred pork and then roughly chop it into smaller pieces. Transfer to a serving dish and top with ½ to ¾ cup cooking liquid from the slow cooker.
To remaining juices, add beans and mash slightly with a potato masher until slightly thickened. Cook until heated through, about 5 minutes.
Use a slotted spoon to scoop black beans onto a tostada shell. Top with shredded pork.
Mix salad mix according to package directions and divide evenly onto the tostadas. Top with additional taco sauce if desired.
