A blind copy or BCC email is a message that you are sending to multiple people but you do not want the other parties on the email to know who that same email is going out too.
To set up a BCC email.....
Start a new email. Under the "TO" tab, there is a "CC" tab. Click the "CC" tab.

This will open your address book.

Put the email address in the "To" section for those the email is being sent directly. The "CC" is for anyone you want copied on the email for documentation purposes, but the email is not really directed at them. The "BCC" function is for the blind copy.
Your address book will populate or you can manually enter an email address. To Blind Copy a person, put their email address in the “Bcc”.
Sending a BCC email will not allow anyone who was sent the email under the ‘TO” or the “CC” to see the “BCC” email address. So, if I send an email to Bobby, Tony, and Stan, but don’t want Bobby and Tony to see that I sent Stan the email too, I would use the “BCC” feature.