Frost is a common problem with wild blueberries and is the primary cause of a poor harvest. Not all blueberry farms have the same climate. The fields in Quebec and Maine blossom before the fields on Prince Edward Island and are therefore more susceptible to frost. The later blossoming of Island berries usually occurs after the last frost of the spring.
According to a study done in Nova Scotia by Agriculture and Agri-Food Canada, flowers that are open or about to open suffer severe damage when the temperature is below -3.5°C for more than two hours. A study in Maine found that although floral buds can tolerate temperatures below 5°C, flowers that are fully open are vulnerable to temperatures below -2.2°C.
There are two types of frost which can damage the wild blueberry: radiative and winter.
Radiative frost occurs when there is little wind and no cloud cover and can occur anytime during the growing season. When there is no cloud cover, the heat escapes into the upper atmosphere and is not reflected back to the Earth.
This cooler air is heavier, and gravity will pull it down slopes to the lowest point of the field. Most blueberry fields are surround by windbreaks and these serve to block the air flow. The trapped air will then cause frost pockets. (Aaswath Raman is his Ted Talk explains this phenomenon. You can watch it here.)
Winter frost, as the name implies, occurs during the winter months when there is not enough snow to cover the blueberry plants. Rhizomes and stems that are in winter dormancy suffer frost damage at around -25°C.