Mince garlic and finely chop enough onion to measure 1/2 cup. Cut enough kernels from cob to measure 3/4 cup.
In a small heavy saucepan with a tight-fitting lid bring water to a boil with garlic, onion, cumin, coriander seeds, and salt. Stir in rice and cook, covered, over low heat 15 minutes. Stir in corn and cook, covered, 3 minutes, or until water is absorbed and rice is just tender. Fluff mixture with a fork and transfer to a large bowl to cool completely.
Rinse and drain black beans. Stir cilantro into rice with beans, lime juice, and salt and pepper to taste.
