Blackberries can be rooted by sticking the tip of a cane in soil and letting it root. I have not done it, I'm sure there are Youtube video explaining it and also on rooting roses from cuttings. I would look at several of them, lots of Youtube videos from people with little knowledge so watch the ones that look legit. I mainly root fig tree cuttings and a few lemon tree cuttings. You can also dig up around a clump of blackberries and separate out plants with roots when they go dormant.