Go to your campaign settings and click 'Templates & Email Sequence' (see: How do I return to my email templates?). Then click '+ Add Attempt #1' and you’ll be taken to your template creation screen. To the right of the 'From' line, you will see the blue text 'Add Cc' and 'Add Bcc'. Click 'Add Bcc' and an additional line will be added to your template for a Bcc address.
Alternatively, you can Bcc an email address to all emails inside the project settings: