BlueMail supports the use of BCC (Blind Carbon Copy). In the Compose View, you can access BCC options by selecting the 3-dots menu or the Cc/Bcc field. This action will open additional lines for CC (Carbon Copy) and BCC where you can add recipients.
To make these lines visible by default, go to More (ooo) | Accounts. Tap the account you're using, then navigate to Advanced Account Settings and enable Always Show CC/BCC Lines in Compose.
Note:
You can also set up your account to add yourself or another email address to BCC automatically.