I think it is cool software that you want to make. I also think the public key distribution via the blockchain is cool, but you will still need some kind of key distribution mechanism especially since PGP keyring servers are not operational anymore. I have seen such platforms without using the blockchain but it would be interesting to see how you do it incorporating the blockchain. I am very much interested to know how you will be storing the public key into the blockchain, in which format you will do it and what key pairs you will be using eg, normal PGP key pairs or commercial certs, key strength, algorithms etc.