Generally, to gradually, and permanatly, lower the pH of soil, you add organic material, that decomposes, it more acid compounds -- things like manure, compost and the like. The garden store sells them.
To lower the pH of irrigation water, quickly, you add mineral acid. Any one will work, just a tiny amount will lower the pH of plain water significantly --sulfuric is fine, nitric is nice, because plants need nitrate. The tiny amount of hydrochloric needed wouldn't likely hurt plants, but over time, the buildup of chloride ions will lead to a saline environment.
Keep checking the pH of the soil as you work it. You want the pH as close to 5.0, or even higher, as possible. The lower the pH is, the better blueberries will grow. Yes slightly lower than 4.5 will help buleberries grow better. But the berries they produce in very low pH soil will be too sour to be edible. I've experienced that firsthand.