I check your backend, you set Editor-TinyMCE as a default editor. So i check your Editor-TinyMCE plugin and found out the access only for Super User : http://screen.stackideas.com/2015-02-25_1639.png
I have change the access to Public and the Registered User can create a blog post. Can you verify this issue has been resolved or not?
I am sorry. I unable to replicate this issue on your site as I have successfully create 3 blogposts. Here is my screenshot: http://screencast.com/t/GJEByaZK . Am I missing anything here?