I am currently using this post to test the ScribeFire add-on (Firefox extension). It is used to publish to various different blogs. It also works if you have WordPress on your own domain. But it also works for WordPress.com. Currently, it apparently does not work with Tumblr due to an API change in 2014.
I’d like to use ScribeFire to post the same message to multiple blogs. I am still trying to work out how to do this. So far, I added two blogs. But when I truy to select both blogs so I can send to both, it is only letting me pick one of the blogs. So I am going to have post here in Writing Shares first and then see what happens on the next page after publishing the post. In the ScribeFire options, it said you can see a list of other blogs to publish to after publishing a new post.
Update: Yes, after publishing on the first blog, you will see a list of your other blogs, with a checkbox by each blog name. You can then check off the boxes and click a button to publish. What I am not sure about is how many blogs you can add. I haven’t found that information yet.