Do you need to? Around here our soil is already pretty acidic, so I don't add anything to acidify it. If you don't know what your soil pH is, you want to do a test or send a soil sample to the nearest place that does soil tests near you before adding amendments.

I agree with nhbabs...in New England we tend to have acidic soil which blueberries prefer..they grow in the wild everywhere. Get your soil tested to confirm that you do have acidic soil (there are pockets of alkaline soil around, for example areas with limestone) and if you do, then you should be fine. Just mulch with compost and keep their roots moist - their roots are near the surface so it's important to keep them well watered for the first fedw years.

I add acidic composted organic matter - pine needles, etc. to the soil. That is enough in our New England soil (unless, as was said, you're on a limestone pocket).