Rinse the soaked black beans thoroughly under cold water, then add them to the slow cooker along with the vegetable broth. This creates a hearty base for the soup.
Dice the onion into small pieces and add it to the slow cooker. The aromatic base will start to develop as it cooks.
Mince the garlic cloves and stir them into the slow cooker. The garlic will release its fragrant aroma as it cooks.
Sprinkle the ground cumin over the ingredients and give everything a gentle stir. Toast the spices briefly by setting the slow cooker to high for 2-3 minutes, until fragrant.
Add the can of diced tomatoes, including juices, to the slow cooker. Stir to combine all the ingredients evenly.
Cover the slow cooker and cook on high for 6-8 hours or on low for 8-10 hours, until the beans are tender and the soup has thickened into a velvety texture.
Once the beans are tender, use an immersion blender directly in the slow cooker to blend the soup until smooth and creamy. Alternatively, transfer the mixture to a blender in batches and blend until silky.
Taste the soup and squeeze in the juice of half a lime, stirring well to brighten the flavors. Adjust salt and spices as needed.
Ladle the soup into bowls, garnish with fresh herbs or a dollop of sour cream if desired, and serve hot for a cozy, nourishing meal.
