TypeApp supports BCC. In compose mode, tap the vertical 3-dot menu and select Add Cc/Bcc. Additional lines for CC and BCC will show where you can add recipients.
You can also set these lines to be shown as default by using More (o o o) | 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.