It wont show their blog posts if the profile is set to 'Private'. You can check that by going to the profile as the admin, then click on the settings 'gear' icon. If their 'profile active' is off, or 'profile privacy' is set to PRIVATE.
The docs for jrCore_list are here:
"{jrCore_list}"
http://www.jamroom.net/the-jamroom-network/documentation/development/89/jrcore-list
Try this code:
{jrCore_list module="jrBlog" search1="_profile_id = 98" limit="1" order_by="blog_publish_date numerical_asc"}
and the other direction is:
{jrCore_list module="jrBlog" search1="_profile_id = 98" limit="1" order_by="blog_publish_date numerical_desc"}
Those will show by the blog publish date. Or you could use the _created date if you wanted to order by the newest added to the system.
The difference is "user adds a blog set to become visible at christmas, then writes another blog post set to be published tomorrow.", the difference is when the blog is published, vs when the blog is created.