Whisk together lemon juice, olive oil, cilantro, and salt and pepper to taste; set aside.
In a small saucepan, bring 1-1/2 cups water to a boil and add corn. Reduce heat and let corn simmer until tender. Drain corn, reserving 1 cup of cooking liquid.
Bring cooking liquid to a boil and add quinoa and cumin. Cover, reducing heat, and let simmer until liquid is absorbed (about 10 minutes). Remove pan from heat and leave undisturbed for 5 minutes. Fluff quinoa with a fork and allow to cool slightly.
In a bowl, combine cooled quinoa, corn, black beans, tomato and onion. Pour dressing over and toss gently to mix. Refrigerate salad until ready to serve.