Mince the garlic, dice the onion and celery, and grate the carrots on a large holed cheese grater. Rinse the black beans in a colander under cool running water and pick out any stones or debris.
Combine the garlic, onion, celery, carrots, black beans, salsa, chili powder, cumin, oregano, vegetable broth, and water in a 5-7 quart slow cooker. Stir well.
Place the lid on the slow cooker and cook on high for 6-8 hours (you want the beans to get VERY soft). Once the beans are very soft, use an immersion blender** to blend the soup until it is thick and creamy (leave some beans whole if desired). Taste the soup and add salt if needed (this will depend on the brand of vegetable broth used).
*Chili powder is a blend of mild red chiles and other spices. It is not cayenne pepper, which is extremely spicy.**If you do not have an immersion blender, wait until the soup cools some and then use a blender to purée the soup in batches. Make sure to never blend hot liquids as they can build pressure in the blender and explode, causing severe burns.