Hold back on adding fertilizer around young transplants. That would likely do more harm than good. There are many reasons why plants are not growing as you might expect. Assuming you did not plant too deep you want to make sure plants have gotten adequate water. The pH is marginal but that alone should not be hindering growth. Are leaves of good color?

I wouldn't even consider the soil pH to be marginal - it is perfectly adequate for growing blueberries. While they prefer a strongly acidic soil, they do not require it. We grow a great many blueberries in my area, with numerous commercial berry farms (including one of the largest wholesale growers of blueberry plants for retail sales in the country) and our native soil is only slightly to moderately acidic (5.6-6.5) - blueberries thrive here without additional acidifying amendments.

Since I do not plant blueberries in the ground, I referred to William D Adams book Growing Fruits and Nuts in the South. He was a county extension specialist I believe from Houston.

He suggested putting out 1 oz per plant of 13-13-13 in March with 1 oz of Ammonium Sulfate(21-0-0) in April, May, and Sept. the first year.

As long as you are using fertilizer that is for acid loving plants (ammonium sulfate) it should keep the pH down in our area unless you are using well water. In Beaumont the bicarbonates, which can bring pH up, is low. Anything less than 92 ppm of bicarbonates is considered low, and anything over 153 is not recommended. Well water may be higher in bicarbonates.

I like to add a little fertilizer once a week. I use 1/2 teaspoon per gallon of water. I now use Peters Professional 21-7-7 for acid special. I got it for about $40 for a 25 gallon bag. It is expensive but less than Miracle gro Miracid.

The reason your blueberries may not be growing well could be aged pine bark fines could have taken some of the nitrogen out of the soil. That should not be a problem this year.

Since your plant have either been in the ground for over 12 months or were from 3 gallon pots, they are going to need nitrogen this year. Usually with bare root fruit trees it is suggested not to fertilize the first year. But with these already being in the ground for 6 months, they will need a little fertilizer.

I know with container blueberries it is recommended to apply ammonium sulfate with water and on the plants when they are moist. Putting the fertilizer out in a dry container can burn the roots.