Drain and rinse black beans from three of the cans. Leave the remaining fourth can undrained.

Heat a large, deep skillet over medium heat then add the olive oil. When the oil is hot, add the onion and cook for 3 minutes, stirring frequently until softened. Add the garlic and continue cooking for an additional 30 seconds.

Add all the black beans, including the liquid from the undrained can. Reduce heat to low and simmer for 15 minutes, until heated through.

Remove from heat, sprinkle with cilantro, and serve warm.