For what it is worth, I've not had any difficulty growing blackberries here in Nebraska and suspect it's all about the cultivars you choose to grow. Some are cane hardy this far north and some are not. Is 'Ebony King' a cane hardy cultivar?
That's often hard information to find because most of them are hardy enough to survive here (and get listed as 'hardy to zone 5'), but some aren't cane hardy and so don't bear fruit. If you end up not being happy with 'Ebony King', you shouldn't give up on blackberries altogether. I can certainly tell you which cultivars have worked for me.
