BlueMail supports BCC. In compose mode, choose the 3-dots menu and select Add Cc/Bcc, it will fold additional lines for CC and BCC where you can add recipients.

You can also set these lines to be shown as default by using More | Various Account Settings | Tap the account | Advanced Account Settings | Always Show CC/BCC Lines in Compose.

Note: You can also Add Yourself to BCC Automatically.