Put the pork into a 6- to 8-quart slow cooker. Add the lime and orange juices.
In a small bowl, mix the zest from the limes, orange and lemon with the garlic, brown sugar, salt, cumin, oregano and red pepper, if using. Rub the seasoning all over the pork. Reserve the lemon.
Cook the pork on low until it is very tender, about 6 hours.
Add the black beans to the pork and cook for about 15 more minutes, to warm the beans through.
Using two forks, coarsely shred the pork. Add the juice of half of the reserved lemon, then taste and add more if you like. Serve with orange wedges on the side, if desired.