Hello,

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.

There is some more information about blackberry knot gall wasp online here and here.

Best regards,