Heat the barbecue grill on medium heat.
For the Nectarines - With the skin still on the nectarines, cut the fruit from each side of the seed in large pieces. Skewer the pieces of nectarines length-wise.
Drizzle agave nectar on the nectarines. Using a cooking brush, spread the nectar on all sides of the nectarines. Grill the nectarines until they have grill marks on each side (about 5 minutes on each side). Remove from the grill and set aside to cool.
For the Salsa - In a medium size bowl, add the onions, jalapenos, and cilantro. Chop the nectarines in large enough pieces so that they are not mushy.
Add the lime juice, salt, and blueberries. Gently mix the ingredients. Add agave nectar (to taste).
Serve as a side dish, on top of fish or chicken, or by itself as a snack.