Depends on the platform you are using, but generally speaking you can write a script that will do that for you.
You can do it on self hosted blogs, but free services will not allow you to do it (unless you bought some $60 software).
You can try using Windows Live Writer: http://windowslivewriter.spaces.live.com/blog/cns!D85741BB5E0BE8AA!174.entry it supports most of popular CMS platforms.
see i think you can do it if you know how to post from email... simply login to your email account write the post and send it to all blogs at once... and it will be posted hope this helps...
may be feed can work. But i am not pretty sure about it. You can have a try. Because i know there is many Automated content blog via RSS feed.
I have a script to post articles automatically to all your wordpress blogs, please pm me if you're interested to buy.