We've added the facility for you to specify a BCC address (or list of addresses) for a personality.
To add a BCC address to your personalities, go to the Options screen, and click on Personalities.

When you compose messages using that personality, the specified addresses will automatically be added to the BCC just before sending.

On the compose screen, any personality that has BCC addresses will have a (b) appended to its name, as a reminder that the message will be copied to additional addresses.

If you wish to override this, you can click on "Change" next to the personality, which will switch you into custom "From" address mode. As you are no longer using a personality, no additional automatic BCC will be performed.