Vasilj, interesting post..! I agree with the sentiment and advice given in Mr. Belling's reply regarding how best to escalate the performance issues of a co-worker. Regarding the blind copy on the emails, I would not want to be treated that way, nor would I treat anyone that way. And, I can say that in over 25 years of using email dating back to mainframe systems, I have never, ever, used the bcc function in an email.
Having said that, I have known some managers in such situations to ask an employee, such as the SME, to document and send an email to the other employee, the PM, with blind copies to management. This is typically when the PM is a problem or border-line employee for whatever reason, and, after numerous difficulties on the project and discussions with the PM giving the PM every chance to improve, the SME is now being asked by management to document and send the PM an email with a blind copy to both managers. As you might suspect, this is more for the purposes of gathering documentation to be used in corrective action with the PM or even termination, as opposed to the initial communication of criticism from the SME to the PM.
I don't like or recommend that as a course of action. Perhaps I am old school, but I like to see co-workers communicate and try to work out their problems before bringing their bosses into it. And, if a worker can't do this, then I like to see that worker communicate their concerns to their immediate manager only. And from that point, the immediate manager owns the problem and, as appropriate, can discuss the SME issues with the manager of the PM - manager to manager. That's what managers are for.
But getting back to the question you raised, "Is this SME's behavior unethical?" In my opinion, yes. And, that is because (irrespective of what PMI or any other organization has to say about ethics) when you do something (with other options available) unto another that you would not want done unto you, you have just acted unethically from a personal integrity point of view. Also, it's bad business. Almost always, at some point in time, people find out when a pointed email to them is blind copied to others. When that happens, like Mr. Belling mentioned, you will have made mortal enemies in the workplace. Even if the SME doesn't care and even if the PM's performance warranted criticism, the resulting mess will likely create more difficulties and workplace disturbances and require more management time and attention than the initial problems that started it all.
Lastly, when you send an email to someone, especially an email criticizing their performance, if no one is copied, the recipient has an expectation that your email is just between the two of you. Manno a manno. The fact is that the recipient has this expectation on account of your actions and that expectation is not true. Purposely, you are denying the truth of the matter that you have copied others on the email. When you think of it in this way, doesn't it seem wrong to do? Send the email just to the recipient or send it to the recipient and copy the managers via cc so that there is no difference between "what you have done" and "what you have led your recipient to believe that you have done."
Great post, I am sure others disagree with me or have different points of view. I hope we hear and learn from them on this topic..!