This is a remarkably good idea. You can store an onion hash in a blockchain transaction and then use the signature of the key which sent that transaction to verify it.

That does seem more secure compared to the way people often talk about onion link lists and search engines.

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.

I assume this is already done, and if not why not?

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?