Exactly what part of such a service would benefit from anything related to a blockchain?
Or whether using certs (really long passwords) is a better option than submitting unhashed passwords on a given datetime to a third-party in order to make sure they're not in the pwned passwords tables?
Blockcerts are for academic credentials, AFAIU.
[EDIT]
Existing blockchains have a limited TPS (transactions per second) for writes; but not for reads. Sharding and layer-2 (sidechains) do not have the same assurances. I'm sure we all remember how cryptokitties congested the txpool during the Bitcoin futures launch.