In a microwaveable cup, combine onion, red wine vinegar, sugar, 1/2 teaspoon of the salt and 1/8 teaspoon of the pepper; cover loosely with plastic wrap; microwave on high power for 1 minute. Let stand, stirring occasionally, until onions turn pink, about 15 minutes.
Meanwhile, preheat grill or broiler. Brush 1 tablespoon of the olive oil on both sides of the salmon fillets; sprinkle with remaining 1/2 teaspoon salt and 1/8 teaspoon pepper. Grill or broil salmon, skin side down, until just cooked through, about 6 minutes.
Divide lettuce leaves among 4 dinner plates; place salmon in the center. With a slotted spoon, remove onions from vinegar; scatter onions, along with the blueberries, over and around the fish. Whisk remaining 2 tablespoons of the olive oil into the vinegar mixture; drizzle vinaigrette over salmon.