Blacklists are available in User level and office-wide accounts. If you are an admin, first navigate to the relevant agents account (ie: who the contact is assigned to/owned by), or the office-wide account.
To see who is on your blacklist, start by navigating to the Settings menu, then click on the Blacklist tab.
Here you will see a list of currently enabled Blacklist rules, a total number of blacklisted contacts, followed by individual details for those contacts included in your Blacklist.
If you want to change any of your Blacklist rules, including adding contacts to your Blacklist, see How do I exclude contacts from all email communications through Blacklists?