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.
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.
Yields 6 servings.