BCC is “Blind Carbon Copy”. It is used for hiding the names and addresses of e-mail recepients. BCC recipients are not visible in the e-mail body or headers! You cannot find this in TMimeMess or TMimePart, because these classes are for creating or parsing the message body and headers.
How to send BCC? Just prepare your mail message without any BCC information. After creation you must send this message to a set of addresses. Just add your BCC addresses to this set. In other words… add your BCC to your SMTP delivery code only! Do not add BCC names and e-mail addresses to any message headers!