Transplant shock is what it sounds like. Make sure you keep it watered well and wait for the shock to subside.

You transplant only when dormant when first go dormant best time in ground better blackberry plants are heavy feeder in pot heavy feeder plants don't grow as good not ample root contact.
A floricane plant impossible to transplant they die after fruit harvest that we see it starts before we see signs.

How long does shock usually take to dissipate? It's still pretty pathetic looking, unfortunately. I knew it was a bad idea...if it croaks, I still have three gigantic babies, at least, that can be transplanted when it gets cold again.

It was growing really well in its pot - 5-1-1; it's huge and has thick, leafy canes...unfortunately I have no way of planting it in ground since we're apartment dwellers.

Shock is just one of those unknowns.....you take two plants and move the.....one never wilts and takes off and the other croaks......you just never know. All you can do is keep it moist and shade might help.