A pH of about 4.5 is required for blueberries to do well. (Some sources say as low as 4.3, some as high as 5.0; most sources say they do well between 4.0-5.5, which is a fairly large range.) At higher pH, the bushes will have an iron deficiency; see the photo and discussion on this page for iron deficiency caused by high pH.
The best way to adjust the pH depends on your soil type and the starting pH. Blueberry plants live a very long time, so it's worth the up front effort to prep the soil so they can thrive. Once they're planted, it's more challenging to alter the pH.
First, get your pH tested. In my opinion, the best thing you can do is to get a professional soil test -- contact your county extension service in the US. When I get my soil tested, the form has a checkbox for blueberries, and the results will come back with recommendations for fertilizer and pH adjustment.
- For a temporary effect you can water the plants weekly with a vinegar solution (2 tablespoons per gallon of water). This won't alter the soil pH though, you need a long term solution.
- You can work in peat moss to make the soil more acidic. On an established planting, you can't work in the peat without damaging the roots. Top dressing with peat will have some effect.
- You can apply sulfur to lower the pH.
- Use a high acid fertilizer when you apply fertilizer. You can find appropriate fertilizer at your garden center. Look for "acid loving plants" on the label; it will usually list blueberries on the back but sometimes the front will just say for azaleas/rhododendrons.
- Top dress with coffee grounds. This will also add some nitrogen.
- Mulch with pine needles, pine shavings, or other acidic mulches.
Once you get the pH to the proper range, you will need to maintain the level. If the area where you live has naturally alkaline soil, it will revert back to high pH over time. Continue to fertilize with acidic fertilizer and use acidic mulches and top dressings.
Certain soil types resist changes in pH more than others. (If you want to get technical, look up "CEC", Cation Exchange Capacity.) Basically, sandy soils are easier to modify. Clay soils are harder. Soils with high organic matter are even harder. As the pH gets lower, it's easier to modify. So if you have sandy soil with a pH of 5.5, you will have to do much less work than if you have loamy clay soil with pH of 6.5. (And this is why I recommend a professional soil test from a local lab -- they can give the best recommendations that are most suitable for the soils in your area.)