Place black beans in a large bowl, cover with plenty of water, and soak overnight. Drain and rinse.
In a large pot, heat the oil over medium heat. Add the diced onion and cook until soft and translucent, about 5–7 minutes.
Add the oregano and salt to the onions. Pour in just enough water to fully cover the onions, then use an immersion blender to blend until completely smooth.
Add the soaked beans to the pot, then pour in the remaining water (and bone broth, if using) so the beans are fully covered.
Bring to a boil, then reduce to a simmer. Cover and cook for about 1 hour, or until the beans are soft.
Use the immersion blender again to blend the beans to your desired consistency. I blend them mostly smooth with a few chunks.
Taste and adjust salt as needed. If the beans are too thick, add a little more water or broth to thin before serving.