An Oregon company is recalling nearly 56,000 pounds of frozen blueberries due to possible Listeria contamination, which federal regulators this week classified as the most serious type of recall.In an online notice, the U.S. Food and Drug Administration said Oregon Potato Company distributed the product in Michigan, Oregon, Washington, Wisconsin and Canada. The agency said the blueberries were not sold to consumers in retail stores. It's unclear whether consumers purchased the product through other channels.Oregon Potato Company did not respond to a request for comment. The company, which produces frozen fruit and vegetable products, initiated the recall on Feb. 12. On Tuesday, the FDA upgraded it to a "Class I" recall, which, according to the agency, means there is a "reasonable probability" that exposure to a product could cause "serious adverse health consequences or death."Listeria can cause mild symptoms such as fever, nausea and muscle aches. More serious cases, however, can be life-threatening.…