I have been using either MiracleGro-acidic, or Schultz Plant Food-acidic, on our blueberry shrubs for 15 years. They both work fine. Here in Madison, Wisconsin, we have hard water, and our soil has pH around 7.6, so getting the pH down and keeping it there is the sticking point for growing blueberries.

I use Miracle grow acid formula as a foliar feed.

I am lucky enough to be in the middle of a commercial BB growing area so there is a excellent dry fertilizer here made just for the BB plants and it is very cheap so win win.

Hollytone is an organic dry fertilizer for acid loving plants.

I primarily use ammonium sulfate to help maintain ph but also use miracid on occassion because of the trace nutrients.

When is the best time to apply the Hollytone or Miracid?

You might check out this article:

http://extension.oregonstate.edu/catalog/pdf/em/em8918.pdf

I generally apply MiracleGro-acidic several times, early in the season. Typically, this would be April 1, May 1, & June 1, approximately. Each application would be one tablespoon, per shrub, dissolved in one gallon of water.