When blood oranges are in season, look for fruit that feels firm and heavy (read: juicy).
On baking sheet, toss oranges, fennel, and oil; season. Roast at 425° for 15 minutes. In small bowl, toss paprika, cinnamon, 1 1/2 tsp. salt, and 1 tsp. pepper; rub all over pork. Transfer to baking sheet. Drizzle with vinegar. Broil until just cooked, turning pork halfway through, about 10 minutes. Top with olives.