(Some raspberries, too!)
Blackberries are at best a rambunctious lot. Large, thick virtually uncontrollable. They even have thorns on their leaves! Come on!
The rambunctious canes easily grow 5' to 7' tall and run everywhere. They are not the well-mannered (in comparison) brambles of my misspent youth, which I have since learned were dewberries. No, these blackberries take being blackberries to new heights of zealotry.
In an effort to control them in the potager, early this spring I cut them back to four feet. The end-result is a massing of berries right at that tip as you can see i the picture below.
Be sure to leave them on the plant until fully ripe so the full sweetness is allowed to develop. The least bit of burgundy red that appears more black than red will leave you with a tart taste and thought about the wisdom of adding these crazed brambles to your disciplined garden.
I highly recommend tying and torturing them to some sort of wired growing structure to keep them within bounds, increase your yields and keep the gardener from becoming a bleeding and scratched victims of its growth!
Their upside? I am growing mine in the shade of an Austrian black pine, fence and garage structures and their don't seem to mind the shade. They take about three years to establish themselves. Once a cane has produced berries, cut it out in preparation for taming the next year's producers.