When you bare root a blackberry plant you lose all floricanes at that time.They be okay next year blackberries don't do good in flower box best to plant in ground so water if needed once a week I'm thinking you water in that box more often.
Your plants loose water picking up roots at transplant and want grow back its part cycle of floricanes fruiting and dying. In zone 8 transplanting time best after first hard freeze like early January.

blackberries need at least 5 gallons of soil to do well in a container. I;m moving my Boysens to 10 gallon just to make it easier in the watering dept. blackberries do not like dry roots in comparison to raspberries.

You might also consider your potting mix. Garden soil isn't necessarily the best choice for containers and can create a variety of problems. If you plan to keep them in containers, you'll find the linked post helpful.

Here is a link that might be useful: Container Soils - Water Movement and Retention

My Triple Crown thornless blackberry canes produce a lot of great berries but I have never learned/figured out which canes should be cut back after bearing & which to leave (oh, I have had so many 'posters kindly give directions ... but to no avail ... I just can't seem to figure it out!!). I think now I will just cut back all the long canes in the Fall & let them grow up the next year! By the time the bearing canes ar finished, I can't tell them then from the canes that did not bear! What a vigorous berry!

Did you have frost which killed the flower.I have the Blackberry with the thorn. It is tardy it takes its time to flower. I am not complaining by doing that they avoid the frost.
The only complaint is my kids and grand kids injure their fingers with the thorns. All my family are saying you should remove these thorny trees but remember I am the tree-hugger I never saw a tree I did not fall in love with. So I constructed a cage of chicken wire around the trees now the kids are unable to reach the trees. They are complaining "We will not be able to get to sweet berries" I told them no worry I harvest, you eat.

Did you have frost which killed the flower.I have the Blackberry with the thorn. It is tardy it takes its time to flower. I am not complaining by doing that they avoid the frost.
The only complaint is my kids and grand kids injure their fingers with the thorns. All my family are saying you should remove these thorny trees but remember I am the tree-hugger I never saw a tree I did not fall in love with. So I constructed a cage of chicken wire around the trees now the kids are unable to reach the trees. They are complaining "We will not be able to get to sweet berries" I told them no worry I harvest, you eat.