Heat the olive oil in a large pot over medium heat, and saute the onion, celery, and red bell pepper until they start to soften, about 5 minutes.

Add the garlic, cumin, smoked paprika, oregano, and red pepper flakes (if using), and stir briefly, just until they are fragrant, about 1 minute.

Add in 1 cup of the vegetable broth, and scrape the bottom of the pot to make sure nothing is stuck to the pan. Add in the remaining broth, along with the drained black beans, and salt. Bring the liquid to a boil.

When the soup is boiling, lower the heat and cover the pot, letting the soup cook until the beans are very tender, about 25 to 30 minutes.

Transfer 4 cups of the soup to a blender. Be sure to remove the vent in the blender's lid and lightly cover it with a towel, to allow steam to safely escape as you blend. (This way the steam pressure won't make the lid blow off the top-- don't burn yourself!) Carefully blend until smooth, then return the puree back to the soup pot.

Add in the lime juice and cilantro, and stir until wilted. Then adjust any seasoning to taste, adding more salt, as needed. (I usually add a ½ teaspoon more salt.) Serve warm with your favorite toppings, like sliced avocado, sour cream, green onions, or tortilla chips. Leftovers can be stored in an airtight container in the fridge for up to 5 days.