The login for ALL hosting control panels is a username NOT an email address.
Please refer to your welcome email for details on your username. If you cannot find your welcome email please contact our technical support team referencing your domain name and we can resend you the login details.
You can use your username on your control panel login screen to use the Forgot Password option in order to reset your password.
note: if you have an email address hosted with Blacknight on cp.blacknight.com then using this as your username will log you in to the email administration section for that specific email address only.