What is this growth on my blackberry bush?
This could be a gall from the blackberry knot gall wasp or a similar species. If it is a gall there are tiny wasps growing inside the structure, which are harmless to people and largely harmless to your blackberry plant. You could, of course, prune this gall out if you wanted to, dissect it if you are so inclined, or just watch it to see what happens.
I've never seen this before- kinda freaked me out. It is good to know the wasps are harmless; but I think I'll prune it nonetheless.
Thanks so much!