Using thin knife, trim silver skin from tenderloin. Mix orange juice, grapefruit juice, cilantro, cumin, oregano, garlic, salt, and hot pepper in gallon-sized zip-top plastic bag. Add pork, close, and refrigerate for at least 30 minutes and up to 4 hours. Meanwhile, make Rice and Black Bean Salad.
Prepare outdoor grill for direct medium-hot grilling. For a gas grill, preheat grill on high. Adjust temperature to 400°F. For a charcoal grill, build fire and let burn until coals are covered with white ash. Spread coals and let burn for 15-20 minutes.
Lightly oil cooking grate. Remove pork from marinade, drain briefly, but do not scrape off solids. Place on grill and cover grill. Cook, turning occasionally, until browned and instant-read thermometer inserted in center of pork reads 145 degrees Fahrenheit, about 20-27 minutes. Transfer to carving board and let stand 3-5 minutes. Cut on slight diagonal and serve with rice and black bean salad.
Rice and Black Bean Salad
To make salad, bring a medium saucepan of lightly salted water to a boil. Add rice and cook (like pasta) until tender, about 16 minutes. Drain in wire sieve, rinse under cold water, and let cool.
In medium bowl, whisk orange zest and 2 tablespoons orange juice, 1 tablespoon vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Whisk in oil. Add cooled rice, black beans, scallions and cilantro, and mix. Let stand at room temperature for up to 4 hours. (If refrigerated, let stand at room temperature for 30 minutes before serving, or rice will be hard.)