Optional: Preheat oven to 425 F. Place sweet potatoes on a foil lined baking sheet in a single layer, drizzle with olive oil, then sprinkle with salt and pepper. Roast 12-15 minutes, or until soft and lightly browned. Remove from heat, cool then chill in fridge. You’ll be adding to slow cooker at end.
Stir together pork, vegetable stock, onion and bell pepper in slow cooker. Cook covered on low heat for 8 hours or high for 2-3 hours.
Once cooked and tender, use forks to pull apart pork into carnitas. Some liquid is ok, but drain extra if needed.
Carefully stir in roasted sweet potatoes and black beans. Cook a bit longer if needed to heat through. Serve in bowls with cilantro as garnish.
To save time you can skip first step and add raw sweet potatoes into pot when slow cooking carnitas. However you won't get benefit of roasted, caramelized flavor.