Doubt blackberry would hold up as well as other materials, large pith in that stuff. And I would think that you'd want to harvest them a bit larger than other shoot shaft materials. Some time ago I watched a a fella on you tube, think it was tribeofbenjamin talk about them. He talked highly of it and that surprised me. Maybe check into his information. A hardwood forshaft could really help I would think. If you do try I'd be interested in what you think.
Mike