We have introduced a special destination object named Mail has external BCC. This destination object will match any emails which have an external BCC address. The rest is up to you, you can use any kind of Source and Information Type with it:
You can simply Log / Block / Quarantine or Archive any emails which have external BCC. To do this, you should use All Sources object as Source, All Matcher as Information Type, and your desired action (Log, Block, Quarantine or Archive) as Rule Action.
You can create more customized polices on BCC mails just tailored for yourself. (Policy Implementation Basics)
Also, here is the screencast for this tutorial:
Have a good day.