That's the nature of email. When a person is BCC'd, they will only see the To:, From: and CC: fields. The BCC field will hide every single email address, even the recipient's.
From wiki:
There are three ways in which the "BCC:" field is used.
In the first case, when a message containing a "BCC:" field is prepared to be sent, the "BCC:" line is removed even though all of the recipients (including those specified in the "BCC:" field) are sent a copy of the message.
In the second case, recipients specified in the "To:" and "CC:" lines each are sent a copy of the message with the "BCC:" line removed as above, but the recipients on the "BCC:" line get a separate copy of the message containing a "BCC:" line. (When there are multiple recipient addresses in the "BCC:" field, some implementations actually send a separate copy of the message to each recipient with a "BCC:" containing only the address of that particular recipient.)
Finally, since a "BCC:" field may contain no addresses, a "BCC:" field can be sent without any addresses indicating to the recipients that blind copies were sent to someone.