Rinse the dried black beans thoroughly and soak them in water overnight or for at least 8 hours to soften and release their nutty aroma.
Dice the yellow onion finely and mince the garlic cloves. Set both aside for sautéing.
Heat a tablespoon of olive oil in a skillet over medium heat. Add the diced onion and sauté until translucent and slightly caramelized, about 5 minutes, releasing a sweet aroma.
Add the minced garlic to the skillet and cook for another minute until fragrant, stirring constantly to prevent burning.
Transfer the soaked beans to the slow cooker. Pour in the vegetable broth, then add the sautéed onion and garlic mixture. Stir in the toasted cumin and smoked paprika if using.
Cover the slow cooker and cook on low for 8 hours or on high for 4–5 hours until the beans are tender and the flavors meld, filling your kitchen with a warm, smoky aroma.
Once cooked, use an immersion blender to puree the soup directly in the slow cooker until smooth and velvety, or transfer in batches to a blender and blend carefully with hot liquid.
Add a squeeze of fresh lime juice to the blended soup, stirring well to brighten the flavors. Taste and adjust seasoning as needed.
Pour the soup into bowls, drizzle with a little olive oil for shine, and serve hot, garnished with cilantro or sour cream if desired.
