Check the pH to make sure it's slightly acid (6 to 6.5). Yellowing can occur when the soil is neutral to alkaline. Also, make sure the plants have enough nutrients. If you've had a lot of rain, the nutrients (especially nitrogen) can wash out of the soil. A soil test will tell you both pH and soil fertility. You can then amend the soil and fertilize if needed. Also, although you can do your own soil test, your local cooperative extension can do one pretty inexpensively and also give you recommendations for your blueberry plants.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine