I've been there, done that with the cardboard thing and I can tell you that roots will definitely NOT grow through it the first year. For annuals this was completely detrimental. For blueberries I'm not sure, if you provided enough fertility on top of it they might do OK the first year and then be able to grow through it the second. Then again it might completely stunt their growth, cause them to fail to pereniallize and set you back to square one the second year.
I would strongly consider removing it. I've had good luck making raised beds just by letting the grass grow a couple feet tall, cutting it as low as possible with a sickle, laying it flat on the ground and building the raised bed on top of that. As long as the raised bed is about a foot high or more and you plant into it immediately, no grass should get through, no cardboard needed.
I like your stone raised beds by the way! Very rustic.