Is an open letter just a blog post you start with "Dear such-and-such"?
Please stop saying open letter. Just delete the "Dear such-and-such" part and remove "An Open Letter to such-and-such" from the title. Now look, it looks just like a normal blog post minus the pretence.
The "classical" definition of an open letter is a correspondence from person/entity A to person/entity B, which is published for general viewing rather than mailed. The goal of such things is to demonstrate a point of view, and action on that view, for others. As a form of rhetoric it is reasonably persuasive if done well.
So to answer your rhetorical question :-) of "Is an open letter just a blog post you start with 'Dear such-and-such'?", Of course it is, it is a rhetorical argument made in the form of a correspondence. Making such arguments in the form of an essay or allegory are also common.
I don't think it's much of a rhetorical device because you could replace every "you" with "they" and lose no impact. The rhetorical device is a meta one, in the sense that there was something novel about a public communication at one point, but now all communications are essentially public. Another key difference is that the recipient of the open letter would have presumably read it, whereas in this case, Google, clearly, ain't gone read it, or even know about it.
Please stop saying open letter. Just delete the "Dear such-and-such" part and remove "An Open Letter to such-and-such" from the title. Now look, it looks just like a normal blog post minus the pretence.