Drain and thoroughly rinse the black beans and set aside. In a large pot, heat the oil over medium heat, then add the onions and saute until soft, around 5 minutes. Add the garlic and continue to cook for an additional minute.
Pour the onion and garlic into a blender, then add the black beans and the vegetable broth and puree until smooth. Using a medium-mesh strainer, strain the mixture back into the pot and discard the remaining pieces of black bean casings.
Heat the mixture over medium heat, then add the corn, epazote leaf, salt and pepper and stir gently for 10 minutes, then serve with a generous sprinkling of condiments.
Epazote may not be found everywhere, so an acceptable substitution would be a sprig or two of fresh cilantro. It can be left out altogether, however, if wanted.