Place the whole pork tenderloin on the unheated rack of a broiler pan. Broil 4 to 5 inches from the heat for 20 minutes (or grill the meat in a covered grill over a drip pan surrounded by medium coals for 30 to 45 minutes) or until meat thermometer registers 160 degree F; turn occasionally. Remove the pork to a serving platter and keep warm.
Meanwhile, in a large skillet over medium-low heat, melt butter. Add onions, salt, and pepper and cook for about 10 minutes or until onions are golden, stirring frequently.
Stir in sugar and cook for 3 minutes more, stirring frequently. Stir in port, sherry, or chicken broth and the vinegar; bring the mixture to boiling. Boil gently, uncovered, for 3 minutes. Stir in the blueberries and halved cherry tomatoes. Cook until mixture is heated through. Remove from the heat.
Cut the cooked pork tenderloin into thin slices. Serve with the blueberry-onion sauce. Makes 4 servings.