> their content blobs are shared

Doesn't this increase the chance of data loss? If a blob gets corrupted, then all the backups referencing that blob will have the same corrupted file(s). This is similar to having a corrupted index in an incremental backup chain (or maybe in this case you would lose everything?), but in the case of incremental backups the risk is mitigated by periodically performing full backups. Also my gut feeling is that you will save space with content-addressed backups only if you're backing up multiple machines that share files, but in the tipical average user scenario where one is backing up a single PC you get a similar space usage. Keep in mind that you tipically delete bacups older than a certain threshold. Could you maybe comment on my points?