@tali1
Buenchico's answer is what you should do *next time* but it sounds like you are asking us how to Bcc something 'after the event' and I assume that you wish to do this without the original addressee receiving a second copy off the email, which might give the game away.
Hopefully, Buenchico's method will work but you could always 'forward' the original message to the intended Bcc recipient.
Back in the days of Outlook, it was possible to see who a message had been Bcc'd to, so there was no privacy element to it. I'd like to know when and why this changed.