Can I publish the reviews I write? This is a follow-up question to Open versus Blind reviewing process, and is somehow related to What happens to the reviews that people write for journal articles after they&#39;re sent back to the author?

However, my question does not concern the reviews I receive for the papers I submit, but concerns the reviews I write for papers I have been assigned. Since the whole process is done under confidentiality, it is not clear who owns the copyright on a review I wrote, and what does the review include. 

For instance, let us assume that X is member of a PC of a conference Y, and assigns to me a review of a paper Z, written by A and B (assuming it's not a double-blind). Can I publish on my blog: "Here is my review for Y, asked by X, on the paper Z, written by A and B"? 

I think that there are two points here: 


Is it legal? (for instance, publishing the camera-ready version of a paper might be illegal due to the copyright transfer, would it be also the care here?). 
Is it ethical? (who should I ask in order to do so? X? Y? A and B? everybody?). 


EDIT: There is been several comments/answer wondering why I would like to publish a review I wrote. To give a bit of background of this question, I believe that the current reviewing system, created when the academic community was small and there was no Internet (i.e. no easy access to information), might not be the best, although clearly working. This is for me a very interesting debate, but somehow out of the scope the Q&amp;A format of Academia SE, which is why I tried to focus on my question on whether it was possible to do so, not if it was the best thing to do in the current system (and just to be clear, I don't plan to do it, but I just like to know what are my options). Anyway, thanks for the answers bringing a different light on this debate. 

EDIT 2: After seeing the update in Jeff's answer, I just realised that I didn't make it explicit that I was talking about reviews after the reviewing process. Jeff says that it's ok if the paper is accepted, and although I clearly understand the argument of why I shouldn't publish a review of a rejected paper, the question still holds: by publishing a review of a rejected paper, I publish the information that these authors submitted this paper to this conf/journal, which is supposed to be confidential between the authors and the editors. Would I break any rule by doing so?
The standard rule in my community is that once I finish reviewing a paper, I'm supposed to pretend that I don't know the paper exists.  In particular, I am not supposed to use any insights I gained by reading that paper in my own research.  I am not supposed to reveal the results to my colleagues.  Some venues ask that I destroy any copies of the paper I'm reviewing, along with any programs or data I used to verify the paper's results.  This embargo lifts only when the paper is finally published, but I am never supposed to reveal my identity to the authors, even indirectly.

Under those rules, publishing reviews is completely unethical.  Maybe it would be okay if I had the explicit permission of both the authors and the editor, but I would expect most authors and reviewers to vehemently object.  I would feel weird even asking.

But even under less stringent reviewing rules, I think posting reviews is a very bad idea.  Criticism is best given privately.  One of the purposes of anonymous reviewing is to give authors brutally honest feedback on their work.  Referees can offer direct criticism without worrying that it will harm the authors, and authors are more willing to hear that criticism because they know it will never be public.

Yes, that means authors sometimes get credit for ideas that I suggest in referee reports.  (Most authors are nice and thank the anonymous referee.)  On the other hand, several referees have offered suggestions that have significantly improved my papers, so it all comes out in the wash.

Update: I should add that these ethical constraints attach only to reviews of unpublished papers being considered for publication.  Once a paper is actually published, everyone is free, if not encouraged, to publish their own reviews of the published version.