I don't know if it's the same case for dogs, but allergy blood testing for cats is notoriously inaccurate and a big waste of $$. Skin testing (where the skin is pricked with various potentially allergenic substances) is much better.
....Just found a link that says blood testing isn't so great for dogs either: http://aadconline.com/skinTesting.htm
At the present time, the only reliable way to diagnose environmental allergies (pollen, dust, etc.) in dogs and cats is through intradermal allergy testing, commonly called skin testing. Although allergy blood tests for dogs and cats are commercially available, they are controversial due to their inaccurate results.