Dice the mangos into 1/4-inch cubes, working around the flat pit. Rinse the black beans thoroughly in a colander until water runs clear - this removes excess starch and salt so they don't muddy the salsa.

Place diced red onion in a fine mesh strainer and rinse under cold water for 30 seconds. This tames the raw bite while keeping the crunch. Shake well to drain.

In a large bowl, combine the diced mango, black beans, rinsed red onion, red bell pepper, jalapeno, and chopped cilantro. Toss gently with your hands or a wide spatula to avoid crushing the mango.

In a small bowl, whisk together lime juice, olive oil, salt, and cumin until emulsified. Pour over the salsa and fold gently to coat everything evenly. Taste and adjust salt or lime - the mango sweetness should balance the tart lime.

Let sit at room temperature for 15 minutes before serving, or refrigerate up to 4 hours. The flavors meld and deepen as it rests. Serve with tortilla chips, over grilled fish or chicken, or stuffed into tacos.