Happy Frog says they pH balance their soils to between 6.3 – 6.8. Definitely too high for blueberries. I suppose it's too late to return the soil, but why did you buy it in the first place if you knew it isn't acidic enough?

If you haven't planted your blues yet, I would recommend purchasing some pine bark fines and amending your soil mix with that. These will help give your potting mix additional structure and aeration. Look Reptibark (at the pet store) to get an idea of the particle size you need. The pine bark fines may be marketed as soil conditioner at your local garden store.

If you want to stick to an organic regimen, fertilize with cottonseed meal. You can apply this anytime fron early spring to mid-summer. Cottonseed meal will help lower the pH of your soil. You could also try using Hollytone, as it is made for acid loving plants.

If you are OK with non-organic fertilizers, definitely seek out some ammonium sulfate. This will provide nitrogen and decrease the soil pH faster than the Espoma soil acidifier. You can combine ammonium sulfate with other fertilizers to get some phosphorous and potassium.

If you are hand watering, you might consider acidifying your irrigation water with some vinegar. You'll have to experiment with differing ratios of vinegar and water to get the pH you want. Concentrated sulfuric acid is also an option. It's marketed as drain cleaner or battery acid. If you choose to use sulfuric acid, do so with extreme caution. Sulfuric acid is a strong acid and can cause severe burns. You can do some reading on the fruits forum of Gardenweb to learn about using sulfuric acid and blueberry irrigation.