Preheat broiler. Broil pork, turning occasionally, until cooked through, about 20 minutes. Remove to a platter; cover to keep warm.
Meanwhile, in a large skillet over medium-high heat, melt 2 tablespoons of the butter. Add onions, salt and pepper; cook until onions are golden, about 10 minutes. Add sugar; cook until onions are caramelized, 3 minutes longer.
Add port, balsamic, blueberries and tomatoes; bring to a boil. Remove from heat. Thinly slice pork and serve with sauce.