Drain the black beans, and reserve 1/2 cup of the liquid.
Peel the onion, and cut it in half. Cut half-moons of onions, and cook until translucent (about 3-4 minutes).
Add the beans, the liquid, salt, childi powder, and black pepper. Heat to a boil, then turn the heat to low and simmer for 15 minutes.
As the liquid disappears, mash the beans to create a thick paste.