Hang on. Incoming messages, or outgoing?

I don't think it can be incoming, since there is nothing in a correctly formatted message to tell you that you were Bcc'd. The one you mention, "To, From, Cc etc" can only "see" Bcc entries in outgoing messages.

If a message arrives where your email address doesn't appear anywhere in its headers then you can infer that you were Bcc'd. So you'd have to filter on To and Cc NOT containing your email address. Setting up such a rule might become quite entertaining if you want to be sure it doesn't mention any of your email addresses, assuming you have more than one email account.

If it's an outgoing message, then presumably YOU Bcc'd it and there is no special treatment needed, is there?

My filter dialogue has an option for "Cc" "contains", which would appear to fit the bill. And you seem to have found this one yourself.

Try to think up an example of what it is you want to filter on. I suspect I am not understanding your problem.

Incoming messages only.

My assistant often CC's or BCC's me in messages to 3rd parties. I don't want these messages landing in my inbox, so I shunt them to a subfolder to review later. CC's are easily done, but you are correct that BCC's land in my inbox without any address info to filter by.

I'll take a hack at the idea of shunting letters that don't contain my address in the To or CC. I have many emails on my Tbird client, but since filters seem to be by inbox I should have much problem. Thanks.