Legitimizing the voter is part of the registration. If the potential voter presents proof of citizenship then there is no need for any database. If they don't then they will need to look up their citizenship in some database that is out of scope for this discussion. A database of citizens (e.g. from the IRS) is subject to its own errors and fraud.
Another source of fraud is a citizen living in two locations and registering in both. That can't be stopped with the system I have described.
You still need a database to say that person has already registered, or is even eligible to register, remember there’s more to it than citizenship.
You’re agreeing to all my point. Blockchain is not solving any of the actual in the field being used every election sources of fraud. It’s solving problems we don’t have and not fixing the problems we have. In and of itself it’s not necessarily a bad idea, except it’s not going to reduce voter fraud one bit.