If the concrete posts are fairly new, they may be leaching alkaline stuff into the soil, too. Painting or sealing them with a clear deck sealant might help.
You will have an ongoing battle to keep enough acidifying material around your blueberries. Compost and peat moss are excellent amendments, and fairly inexpensive. If you mulch them with a top dressing of a good amount each spring, and pile leaves around them as mulch in the fall you may keep up with them. The fall mulch might help to keep them from dessicating during the winter, too.
But the problem later becomes when the bushes get larger, and their roots go deeper and wider, they will eventually reach the native soil. Then you will be seeing the unhealthy results of growing an acid-loving plant in soil with a too high pH.
