If you received the e-mail, there was never a Bcc header in it to include in
a print out.
If you sent the e-mail, the print of it only includes your name and the
From, Sent (Date), and To headers. No Bcc gets included.
If you need the list of Bcc recipients in your sent e-mails, you could
simply open the Sent Item copy (double-click to show in its own window) and
view the Bcc field. That won't help if you're looking to include a hard
copy of the Bcc list in your printed copy of an e-mail.
For your sent e-mails and to get the Bcc field included in a printout, it
looks like you need to edit the form used to show the read-view of an item.
Google still works:
Designing a form isn't that easy. You'll have to drag around or resize
existing controls to make room to add another one (for the "Bcc:" text
object as a title and the value object to show the list of Bcc recipients).