Once your blackberry plants grow new shoots that develop some leaves and are a foot or two tall, that will prove out the viability of the clump. It would then be fine to prune out some of the old original canes and leave a couple to bloom and fruit. Blackberries in good soil and properly sited are vigorous plants and can tolerate a lot of literature ignoring. If for some reason the old canes do not bloom and the new canes get pretty tall and leafy, then cut out all the old canes.

Blackberries: Cutting back during the growing season will cause the remaining cane to branch out and that increases berry production next year. You can prune at any time to a size that is convenient for you. In some locations the canes will still be growing in the fall; pruning the main cane then will result in shorter branching or no branching.

It sparks a couple of questions. The plant will put out new canes whether or not the old canes are pruned. It looks as if I will be taking the canes that are already there from this year and bracing at about 6' high horizontally along the garage. I figure that I can continue to use these canes year after year, not unlike grape vines. It looks as if sooner or pater, I may have to clip these out and use new ones as teh central feed. It will be interesting to see how the new canes are produced. Since they are flimsy at best, I figure they will drop down. The new lateral growth of tip-pruned canes will be at random angles but does not grow "down" from the cane tip, but sideways from multiple points. In this case, sideways is up or down, no? Prune out old canes after harvest and tip prune new canes when maximum desired height is reached. In this case, maximum width. Are you going to attach the clips directly to the garage wall? That would require over 100 clips that would need to be removed and re-placed every year. I can imagine using no more than 6 clips as the one in the picture is held up, although sloppy, using only three. I never recommend training blackberries directly against a wall and ripe fruit will stain it. Go with the t-posts in the ground a couple of feet from the wall and attach canes to them and to each other. I can see your point but I can only imagine that the canes will grow against the garage anyway without considerable maintenance anyway. I am not all that attentive of a gardener in the first place so it is going to happen any way. Still, no one has indicated any method to actually hold the horizontal canes in place for support. I can use steel poles with some kind of wire to wood beam across but am still at a loss for actual attachments.

Those thorns will take a long time to break down in a compost bin. It will happen eventually, but in the meantime you'll have to work around them when you harvest the finished product. As a rule of thumb I don't add any thorns to composters because I'd rather avoid surprise stabbings, even while wearing gloves. Piling up those thorny canes would provide excellent cover for wildlife. Left undisturbed, reptiles or small herbivores like rabbits would turn it into their condo. If wildlife gardening is not on your agenda, consider using the canes as mulch. When I prune anything that is not suitable for either a composter or the wildlife brush pile, I toss it under a large Lady Banks rose where a neat appearance is not a priority.