Slice the sweet onions into ½-inch thick rounds. Lightly brush the onion slices with oil. For grilling and cutting ease, try to keep the onion slices together in rounds. Grill the onion rounds for 4–5 minutes on each side, depending on the heat of your grill. They should have grill marks and be softened through to the center rings. Remove the onions and let cool.
Combine the blueberries, orange juice, vinegar, pepper, honey and salt in a small bowl. With the back of a fork, gently crush about 8–10 berries which will help to infuse the juices with the blueberry flavor. Let stand for about 30 minutes.
Chop the cooked onions into ½-inch pieces and place in a mixing bowl. Add the macerated berries, olive oil, cilantro and green onions and toss just to combine. Taste and adjust seasonings accordingly.