Allowing All Users to Post to the General Blog
Using Jamroom
Need a small shift in thinking.
Right now your thinking you want a profile that all members can add to the blog. Instead approach it from the perspective of "I want a single location where all blog posts can be seen" and what you're after becomes real easy.
This doc has some useful info related to levels:
"concepts the admin user needs to know"
https://www.jamroom.net/the-jamroom-network/documentation/getting-started/2167/concepts-the-admin-user-needs-to-know
So instead of trying to put 'everybodies blog on one profile' create 'a location that shows everybodies blogs'
Example:
https://www.jamroom.net/blog
It can be done easily at site level using either {jrCore_list module="jrBlog"} or with Site builder
"{jrCore_list}"
http://www.jamroom.net/the-jamroom-network/documentation/development/89/jrcore-list
"What is Site Builder?"
https://www.jamroom.net/the-jamroom-network/documentation/site-builder/2634/what-is-site-builder