First @VladislavRastrusny is right.
This is not a pattern problem, a pattern is a general abstract way of solving an abstract known problem, that you then (try to) apply to a concrete case.
Here @JohnLBevan you seems more to have difficulties transforming an app/service/platform idea to "reality".
So to follow my comment and elaborate:
Blockchain at is core is a decentralized, distributed and hard to temper(so secure to some goood enough extend) database system.
If every user as his own block with his informations, and the "app" let him(or other service/app on his phone/computer automaticly) search for other people information.
The "link" or "transaction" part can be used to give access to information, so in our situation if Bob authorize Alice to have his informations we have transaction et Bob and Alice are linked.
Blockchain as a system call PoW(Prove of Work) and can garantie thzt only Bob is able to alter his information, therefore everybory in the network can be pretty confident that they'll not be another person pretending to be Bob.
Bob is only responsible for his entry and the validity of it. When Alice wants to, for example, call Bob, the app search for the Bob block and if it is considered valid(throught PoW) by the system transfer the phone number to the dialing app, and voila.
Of course this only works if Bob has an up-to-date entry but this problem is already here, and by having to mainting only your entry, the cost is fairly low, that's the all point right.
This is why your idea is great, because your app can become a decentralized, distributed IAM system(you'll need to secure the access to users devices with 2FA).
Note that there is an alternative to Blockchain called Hashgraph you may want to look into that too.
Anyway, it will not be easy by any mean, so you'll need to surround yourself with the best people you can.
But again it seems like a great and ambitious idea, so I wich you good luck.