A quick google revealed that there are indeed Emacs lisp hacks that allow one to write blog posts in the comfort of all things Emacsy.
One of them, which I'm currently testing, is available at http://code.google.com/p/e-blog/. We'll see if this gets posted, and how the formatting turns out. A quick look at the Emacs lisp code suggests that while this text is auto-filled as I write it, e-blog will reformat it before submitting it to blogger.com. Well, as said, we'll see.
UPDATE: The post got happily through, as you can see. The lisp code seems to be working just fine for posting -- I'm now testing whether updating existing posts works -- and a one-line insertion to e-blog.el gave me ispell based on-the-fly spell-checking (i.e. I enabled the flyspell-mode minor mode as default for the blog posting mode). Wonderful. No longer must I suffer the horrible Google interface. One less reason to ever leave Emacs.