It makes sense to me to incorporate this functionality into word processing applications like Word, Apple’s Pages and even OpenOffice. Granted Apple has iWeb and that may be the best application to incorporate blog editor functionality into. Short of that happening there are many excellent blog editors out there. My focus is the Mac so I can point you to Qumana (can be slow on old Macs), which is free, and to MarsEdit which you pay for and which is a really fast blog editor.
If you have Word 2007 and have been looking for a blog editor as an alternative to updating your blog then all you may need to do is dig a little deeper into Word itself to find what you are looking for. Everything TypePad has a great post about using Word 2007 as your blog editor (the functionality is built in) which I recommend if you are interested in using what you already may have.