I don't use any. Just use a 511 mix without lime (use some calcium instead,) and just make sure you acidify your water to about 4.5-5 pH.

About 1 cup, so how did the blueberry do over winter? Sorry I didn't see your other post. I don't come here that much.

Soil acidifier for blueberries in pots? What kind of soil acidifier are we talking about? I have 6 blueberries in pots and 2 are quite large in 25 gallon pots, and need repotting. The old mix has broken down. Maybe this is something I can use.

Well sulfur is the best but kinda strong, and you have to monitor PH, Suzy I think is using Espona's Soil Acidifier. it's 30% sulfur, really only 18%. 12% is gypsum. I use sulfur and monitor levels. The reason i use it is here rainwater is neutral, and as the soil breaks down, the pH rises, at least that is why I think it rises? So I use this product myself
http://smile.amazon.com/gp/product/B00HAR7EIC/ref=ox_sc_sfl_title_1?ie=UTF8&psc=1&smid=A1849PG4OXHM2J