So... it sounds like what you're looking for is a password manager.
In theory, you could achieve this by only storing the password DB in a cloud service you access through the Tor browser, but then you run into two other problems. First, how do you remember the address of the service you're using to store your password DB? If you store that address on your device, you've lost your plausible deniability. I guess you'd have to use a well-known clearnet site you access through Tor (Dropbox, Google Drive, etc).
That leaves another problem though. If you have a password manager installed on your PC, that implies the existence of a password database, which might also cause you to lose your plausible deniability.
Maybe if there was a profiles/password manager feature built into the Tor browser that lets you decrypt and load a profile from a service provider, then destroys all local evidence of that profile when you close the browser? Seems like a feature that could be useful in all browsers actually. A cross-browser password manager and bookmarks sync feature? That'd be really convenient.
Also see this blog post: https://blog.torproject.org/cooking-onions-names-your-onions
But the blockchain can still be flooded with fraudulent onions and at some point you're still putting all of your trust into a form of authority.
The practical implementation would be the bookmarking service would authenticate their links against this blockchain, and the user would either have an agent on their system to validate the claims, or another 3rd party who they trusted to validate the claims?