Open the can of black beans with a can opener, then rinse them thoroughly under cold water until the liquid runs clear. Drain and transfer to a large mixing bowl.
Finely chop half a small red onion, then add it to the bowl with the beans. This adds a crisp texture and sharp flavor to the salad.
Cut a lime in half and squeeze its juice directly over the beans and onion. This brightens the dish with a tangy, citrusy flavor.
Add chopped cilantro to the bowl, then sprinkle in ground cumin and a pinch of salt. Stir gently to combine all the flavors evenly.
Drizzle olive oil over the mixture for a subtle richness, then gently toss everything together with a large spoon or spatula until the beans are coated and the ingredients are well mixed.
Taste the salad and adjust the seasoning with more salt or lime juice if needed. Let it sit at room temperature for about 30 minutes to allow the flavors to meld, or refrigerate if you prefer it chilled.
Give the salad a gentle toss again before serving to redistribute the dressing. Serve the black bean salad chilled or at room temperature, garnished with extra cilantro if desired.