Add the black beans, diced tomatoes with their juices, vegetable broth, onion, red bell pepper, green bell pepper, and garlic to your slow cooker.
Stir in the chili powder, cumin, oregano, salt, black pepper, and cayenne pepper (if using) until everything is well combined.
Cover and cook on LOW for 7-8 hours or on HIGH for 4-5 hours, until the vegetables are tender and the flavors have melded together beautifully.
About 30 minutes before serving, use a potato masher or the back of a spoon to mash about 1/3 of the beans against the side of the slow cooker. This creates a nice, thick texture while keeping some beans whole.
Stir in the lime juice just before serving. Top with fresh cilantro, diced avocado, and serve with tortilla chips for scooping.