Blueberry potting mix

The loam component is a good idea as it will provide a more long lasting medium for growing them in - however, the limestone most JI composts contain is not good news at all for blueberries.

There is a JI formulation for ericaceous plants which you may be able to buy, but you could consider making it yourself:

http://gardenofeaden.blogspot.co.uk/2011/03/how-to-make-john-innes-compost.html

If you don't want to go to those lengths, you could just add some ordinary soil to the ericaceous compost, as long as you are certain it isn't at all limey (pH 7 at the highest, preferably lower.)