Remove the venison from the refrigerator and allow it to temper at room temperature. Light your grill and set it up for two zone cooking. Establish temperature at 225°F.
While the grill is preheating, rub the venison with olive oil and season with rosemary salt and pepper. Insert leave in thermometer into thickest portion of meat, and set aside till the grill is preheated.
Place the rack of venison on the grill over indirect heat. Close the lid and allow the venison to cook till it reaches 115°F.
While venison is cooking, heat oil for the sauce over medium high heat in a medium saucepan. Add the diced onions and cook until translucent, about 5 minutes. Scrape up any browned bits from the bottom of the pan, and add the remaining ingredients. Bring to a boil while whisking, then reduce to a simmer and allow the sauce to thicken.
Remove the rack of venison from the grill when it reaches 115°F. Tent with foil and allow it to rest while stoking the coals to achieve high heat.
Place the venison back on the grill over direct heat. Sear all sides of the venison for a total of five minutes, turning often in order to achieve an overall char.
Remove the venison from the grill, slice and serve with warm blueberry sauce.