Yes but it also makes it a LOT less likely that your passphrase would be compromised because it is never sent to any service over the wire; with blockchain sign

Yes but it also makes it a LOT less likely that your passphrase would be compromised because it is never sent to any service over the wire; with blockchain signatures, your passphrase never needs to leave your own machine. So the service providers you log into never have any opportunity to even see your passphrase; they only see your signatures which proves that you know the passphrase. You could use a different program for signing login messages so that you don't even need to insert your passphrase inside third party UIs. You'd use one trusted program of your choice to handle logins for all the different services; just copy paste the signature into the different services. There are some stateful blockchains which allow changing the passphrase but of course there is no 'forgot my password' feature; a malicious actor who manages to steal your passphrase could potentially use it to change your passphrase and lock you out of your account but at least you don't have to live in doubt if you suspect that your passphrase was compromised. The idea behind the blockchain auth approach is that you would only need one passphrase for all services but you would have to secure it carefully since losing it would cause you to be locked out of all services associated with that account.