A blockchain can't prove that you created anything. It can only prove that you have possession of the same private key as a person who had knowledge of some information at a certain time in the past. Having knowledge of something isn't the same thing as creating it, and furthermore possession of a private key isn't proof that you are the sole or original owner of that key. So, I fail to see how a blockchain could prove ownership of any copyright. Furthermore, proof of ownership is rarely the main issue in copyright disputes, so even if a blockchain did work to prove ownership of copyrights it wouldn't even be useful.