How to Change "USER has no blog entries" Text
You have to isolate first exactly where this text is coming from. From your description, it sounds like you're using the core blog module which has this line here:

https://api.drupal.org/api/drupal/modules%21blog%21blog.pages.inc/function/blog_page_user/7

else {
  drupal_set_message(t('!author has not created any blog entries.', array('!author' =&gt; theme('username', array('account' =&gt; $account)))));
}


So the correct string you want to use (and what your string override in settings.php will be looking for) is 

!author has not created any blog entries.


Using the string overrides module or your settings file, its easy then to change this text to whatever your like