Pour the black beans into a fine-mesh strainer or colander. Rinse thoroughly under cool running water until the water runs mostly clear, then drain well. Excess water can dilute the dressing.

Make sure the frozen corn is completely thawed: Drain away any collected moisture and pat it lightly with a clean kitchen towel or paper towels if it seems especially wet.

Halve the tomatoes and dice the red bell pepper and red onion. Chop the cilantro. Keep the pieces relatively small and even so each spoonful contains a balanced mixture.

In a small bowl or measuring cup, whisk together the olive oil, lime juice, garlic, cumin, salt, black pepper, and optional cayenne. The dressing should look lightly emulsified, with no large pools of oil.

Add the black beans, corn, tomatoes, bell pepper, red onion, and cilantro to a large bowl. Pour the dressing over the mixture.

Fold the ingredients together until everything is evenly coated. Avoid vigorous stirring, which can crush the beans and tomatoes.

Cover and refrigerate for 20 minutes. This short rest allows the beans and vegetables to absorb some of the lime and cumin flavor.

Just before serving, gently fold in the diced avocado. Adding it at the end helps the pieces retain their shape and color.

Stir the salad gently and taste: The finished salad should be bright, lightly tangy, and well seasoned without excess liquid pooling at the bottom. Serve chilled or cool.