Open the can of black beans using a can opener, then rinse them thoroughly under cold water until the water runs clear. Drain well and transfer to a large mixing bowl.
Finely chop the red onion, aiming for small, uniform pieces that will blend smoothly into the salad. Add the chopped onion to the bowl with the beans.
Roughly chop the cilantro leaves, making sure to include tender stems if desired. Toss the chopped cilantro into the bowl with the beans and onion.
Juice the limes until you have about 4 tablespoons of fresh lime juice. Pour the lime juice over the bean mixture.
Drizzle the olive oil over the ingredients, then sprinkle the ground cumin, if using, and a pinch of salt. Use a spoon or spatula to toss everything together, ensuring the dressing coats all the ingredients evenly.
Taste the salad and adjust seasoning with more salt or lime juice if needed. Cover the bowl and refrigerate for at least 30 minutes to let flavors meld and develop.
After chilling, give the salad a gentle toss and transfer to a serving dish. Garnish with extra cilantro or a squeeze of lime for a fresh presentation.