Supposing you are talking about the System.Net.Mail.MailMessage class, what you need is already provided by the MailAddressCollection.Add method (Bcc is of the MailAddressCollection type).
Just call Add method with multiple e-mail addresses separated by a comma character (",").
Check this:
http://msdn.microsoft.com/en-us/library/ms144695(v=vs.100).aspx