There are container mixes designed for acid soil plants such as blueberries, but they seem to be available only on the West Coast. You can make your own using half peat moss and half screened pine bark. This will need supplemental fertilization since it contains little in the way of nutrients. I've also been using a mixture of Sunshine mix #4 plus added bark for rhododendrons. The Sunshine mix contains quite a lot of perlite and is designed to provide lots of airspace and drainage. The container forum will have other formulas such as the "gritty mix" worth checking out.