Soak the corn in a bucket of water for 15 to 20 minutes.
Pre-heat your grill on high or set your bed of coals. Grill the corn for 10 to 12 minutes, turning the husks as they brown. Check the corn by peeling back the top of 1 ear and test the kernels. It should be plump and juicy, not starchy. When cooked, remove the corn and let cool.
Lightly brush the onion slices with the canola oil. For grilling and cutting ease, try to keep the onion slices together in rounds. Grill the onion rounds for 4 to 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.
Remove the husks and silk from the corn. To de-kernel the corn use a chef knife to cut the corn in half. Place the corncob standing on end on a cutting board. Using a sharp paring knife, run the knife along the surface of the cob removing the kernels as close to the cob as possible. Transfer the kernels to a bowl.
Lay the grilled onion rounds on a cutting board and cut into a ¼–½ inch dice. Add to the corn.
In a small bowl combine the thyme, tarragon, sherry vinegar, lemon zest, lemon juice, olive oil and salt. Add the dressing to the corn-onion mixture and gently toss together. Add cracked black pepper and adjust the seasonings as needed.
Add the blueberries, toss gently and season as needed. It is best to let the relish sit for 15 to 20 minutes before serving; allowing the flavors to blend.
Serve as a condiment for grilled meat or as a salad on a bed of greens topped with some Great Hill blue cheese or goat cheese.

Makes 6 cups.