That's my expectation as well.
Why? Let's assume your blog article is public (visible to everyone): BlogsAggregator rightfully shows it. When you click the article, Liferay tries to find a page where all the proper context for a blog article is found - e.g. the only page where the blogs portlet is on, the user's private page. Private pages are only visible to members of the site they're private to, and in the case of a user's private site, that user is the sole member of their own site.
As blogs are inherently public (unless the content is private - then protect the content), my recommendation is to drop the Blogs portlet on a user's public page.
