You are right on how the blobstore works, it will write new files for each new binary or version. This is a bit greedy in terms of disk, but it is also fast.
When it comes to your issue, it is controlled by the Idprovider if it changes anything on login - looks like it does in your case.
Even if every login creates a new version of the user node, there are no links to a specific version of a user (or other data nodes for that case). Users are actually referenced with a principalkey in the format:
So, I'm not sure what actually causes your problem? Is it likely that the username (loginid) changed?