It's not an online editor, but it is my preferred software for posting to ANY blog, whether it be Wordpress, Blogger, or anything else:Windows Live Writer
In one mode, it's very plain with a white background and black text.
In another, it's like typing directly onto your blog page. It's blog theme aware. (you can't get more WYSIWYG than this)
And in another, you can edit the raw html.
Has preview so you can see how it will look in the blog before posting it. Lots of plugins
It is especially good for inserting images into posts, allowing resizing, effects, watermarking, creating a seperate thumbnail that can link to original, or even a resized image.
Also great for doing some offline blogging and posting later.
This is one of those applications that Microsoft did such a fantastic job on that I am amazed that it's for free.
And it seems that someone has even made it portable