Looks like @gerhard actually committed the fixes. This should prevent staged user creation when the email domain is blacklisted:
https://github.com/discourse/discourse/commit/7f50380221f4a77bec7dfedc141ef99213de9438
We also will no longer staged accounts if the email was rejected:
https://github.com/discourse/discourse/commit/76706f91447de810f6aa3b618a2f5fe8c0924b4b