A recent New York State Bar Association Ethics Opinion (available here) addresses whether a lawyer must obtain the consent of opposing counsel before he or she can blind copy the client on correspondence to opposing counsel. The opinion concludes that "[a] lawyer may blind copy a client on e-mail correspondence with opposing counsel, despite the objection of opposing counsel. Because a lawyer is the agent of the client, sending such a blind copy is not deceptive. However, there are practical reasons why the lawyer should consider forwarding the e-mail correspondence to the client rather than using “bcc”."
Lawyer Ethics Alerts Blog has more information here.