There are a few ways an email address will end up on your block list:
The member or referral unsubscribes from all emails in their email preferences page.
The admin unsubscribes a member or referral from all emails in the member or referral details using the edit button next to "Unsubscribed to All".
An API call is made to the POST API/email/unsubscribe endpoint with the email address to unsubscribe from all.
To view or remove a blocked email address go to Settings > Email Block List or click here.
It is not recommended to remove a blocked email address unless it was blocked in error.